revolution of the earth essay

• Celestial Bodies
• Rotation And Revolution

Rotation and Revolution

We have heard the terms rotation and revolution associated with celestial objects. Let us know more about rotation and revolution and the difference between rotation and revolution.

What is Rotation?

What is revolution.

Rotation of the Earth

Earth rotates on its axis from west to east, and the Sun and the Moon appear to move from east to west across the sky. The spinning of the Earth around its axis is called ‘rotation’. The axis has an angle of 23 1/2º and is perpendicular to the plane of Earth’s orbit. This means the Earth is tilted on its axis, and because of this tilt, the northern and southern hemispheres lean in a direction away from the Sun. The rotation of the Earth divides it into a lit-up half and a dark half, which gives rise to day and night. The direction of the Earth’s rotation depends on the direction of viewing. When viewed looking down from the North Pole, Earth spins counterclockwise. On the contrary, when viewed looking down from the south pole, the earth spins in the clockwise direction.

Importance of Earth Rotation

Some of the importance of the rotation of the Earth are listed below:

• The Earth’s rotation creates the diurnal cycle of lightness and darkness, temperature and humidity changes.
• The Earth’s rotation causes tides in the oceans and seas.

Revolution of the Earth

The movement of the Earth around the Sun in a fixed path is called a revolution. The Earth revolves from west to east, i.e., in the anticlockwise direction. The one revolution of the Earth around the Sun takes around one year or precisely 365.242 days. The revolution speed of the earth is 30 km/s -1 .

Importance of Revolution

• Revolution causes seasons.
• Revolution creates perihelion and aphelion. Perihelion occurs when the Earth is closest to the Sun. Aphelion occurs when the Earth is far from the Sun.
• Revolution has a direct influence on the varied length of day and night time. The duration of days and nights are the same at the equator. This is known as the equinox. The duration of days and nights vary in the Northern and Southern hemispheres. This is known as solstices.

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Rotation and Revolution of Planets

Difference Between Rotation and Revolution

The table given below provides the basic differences between rotation and revolution.

Watch the video below to understand what would happen if the Earth stopped spinning

Frequently Asked Questions – FAQs

What is meant by rotation, what is meant by revolution, do earthquakes affect the earth’s rotation.

Using the data from the Indonesian Earthquake, NASA calculated that the earthquake affected Earth’s rotation, decreased the length of the day, shifted the North Pole by centimetres and slightly changed the planet’s shape. The earthquake that created a huge tsunami also changed the Earth’s rotation.

Has the Earth’s rotation ever speeded up in the past?

Probably, but in the last 900 million years, any speed-ups have been superimposed on a more or less steady slow down in spin rate. Even today, we can identify how the Earth’s rotation rate changes fast and slow by milliseconds per day, depending on how the mass distribution of the Earth and its atmosphere change from earthquakes and the movement of water and air.

Is it possible to slow down the Earth’s rotation artificially?

It is said that humans have made a measurable change in the Earth’s rotation period by several microseconds by accumulating vast reservoirs with trillions of tons of water. There may be a weak interaction between this activity and the weather over the long term, and possibly even in the strength of the Earth’s magnetic field, which is very sensitive to the Earth’s rotation rate.

What is the angle made by the axis of the earth with its orbital plane?

The angle made by the axis of the earth, which is an imaginary line with the orbital plane, is 66 degrees.

What is an equinox?

An equinox is defined as the time when the sun crosses the celestial equator such that the length of the day and night are equal. Every year has two equinoxes. Also, the length of nights at latitudes L degree north and L degree south are equal.

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Earth and Space Science

Earth’s rotation and revolution explained.

• The Albert Team
• Last Updated On: March 6, 2024

Have you ever wondered what keeps the day transitioning into the night and back again? This daily cycle is all thanks to Earth’s rotation, which affects everything from the time zones we follow to the weather patterns we experience. This post will review Earth’s rotation and revolution and illustrate how they impact our daily lives. Understanding Earth’s motion isn’t just about knowing why the sun rises and sets but also about comprehending a basic principle that underpins life on our planet.

What We Review

Rotation vs Revolution: What’s the Difference?

Often, the terms “rotation” and “revolution” are used interchangeably. However, they describe distinct movements of Earth that profoundly affect our environment. So, what is the difference between rotation and revolution?

Rotation refers to the Earth spinning around its axis. Imagine an invisible line running through the Earth from the North Pole to the South Pole; this is Earth’s axis. Every 24 hours, Earth completes one full rotation, which is why we have day and night.

Revolution , on the other hand, is the Earth’s journey around the sun. This path isn’t a perfect circle but rather an elliptical orbit, and it takes about 365.25 days to complete. This revolution is the reason we have seasons.

Understanding Earth’s Rotation

Earth’s rotation is how our planet spins around its axis. Picture Earth like a giant top spinning in space, completing a full turn every 24 hours . This rotation gives us the cycle of day and night—daytime when your location faces the sun and nighttime when it turns away.

Historically, the realization that Earth rotates came from observing the sun’s and stars’ apparent movement. Ancient astronomers initially thought that Earth was the center of the universe and everything revolved around it. However, figures like Copernicus and Galileo challenged this view, providing evidence that the Earth spins on its axis. The Foucault pendulum , introduced in the 19th century, offered direct, observable proof of the Earth’s rotation. As you can see , it swings in a consistent direction while the Earth turns beneath it.

Earth’s Revolution Around the Sun

Earth’s revolution around the sun describes our planet’s yearly journey in space. This orbit isn’t a perfect circle; it’s slightly stretched, causing Earth’s distance from the sun to vary throughout the year.

This elliptical path brings us closer to the sun at perihelion in early January and farther away at aphelion in early July. This slight variation doesn’t significantly impact our climate. The primary influence on seasons is Earth’s axial tilt. As Earth revolves, its tilted axis leads to varying sunlight angles across the globe, resulting in seasonal changes. For example, when the Northern Hemisphere tilts towards the sun, it experiences summer. Meanwhile, the Southern Hemisphere faces away, experiencing winter, and vice versa.

Earth completes this orbit in about 365.25 days, a fact that defines the length of our year. This is why we have a leap day every four years to keep our calendar in sync with Earth’s orbit.

Real-Life Implications of Earth’s Rotation and Revolution

Impact on navigation.

Historically, Earth’s rotation has been crucial for navigation, especially before the advent of modern technology. Mariners relied on the position of the stars in the night sky to find their way. The postition of the stars change predictably as the Earth rotates, an d seafarers use this to determine their latitude and longitude at sea. Even today, with GPS technology, understanding Earth’s rotation is vital as it influences the positioning of satellites and the accuracy of GPS signals.

Influence on Timekeeping

The concept of a day is directly tied to Earth’s rotation. Our 24-hour day is Earth’s period to complete one full rotation on its axis. Time zones are determined by the Earth’s rotation relative to the sun, dividing the planet into different time zones based on longitudinal degrees. As Earth rotates, different parts of the world experience sunrise and sunset at different times, leading to various time zones.

Effects on Technology

Earth’s rotation and revolution have significant impacts on technology, particularly in the fields of telecommunications and space exploration. Satellite communication systems must account for Earth’s movement to maintain alignment and provide consistent coverage. Astronomical observatories and space launch facilities must also consider Earth’s rotation when planning observations and launches.

Frequently Asked Questions About Earth’s Rotation

What causes the earth’s rotation.

Earth’s rotation is due to how it formed and the conservation of angular momentum. When our solar system formed, the cloud of gas and dust that became the Earth was spinning, and as it condensed to form the planet, it retained this spinning motion.

What is Earth’s rotational speed?

Earth’s rotational speed is fastest at the equator, reaching about 1,670 kilometers per hour (or around 1,040 miles per hour). It decreases as you move toward the poles, where it is effectively zero. This variation affects phenomena such as weather patterns and the Coriolis effect, which influences the direction of ocean currents and wind.

Why can’t we feel Earth’s rotation?

Earth’s rotation is very consistent and doesn’t change speed abruptly, so we don’t feel any acceleration or deceleration. Plus, the Earth is so large that its curvature is difficult to perceive, making the rotation feel even less apparent.

What would happen if the Earth stopped rotating?

If the Earth stopped rotating, it would be catastrophic. One side of the Earth would be in constant sunlight, while the other side would be in perpetual darkness. This would cause extreme temperature changes, disrupt weather patterns, and eliminate the day-night cycle as we know it.

How does Earth’s revolution around the sun influence seasons?

The seasons are influenced by Earth’s axis tilt and elliptical orbit around the sun. As Earth revolves, different parts of the planet receive varying amounts of sunlight, leading to seasonal changes.

Why is a year 365.25 days long and not exactly 365 days?

A year is 365.25 days long. This is the time it takes for Earth to complete one full orbit around the sun. The extra 0.25 day adds on over four years to an additional day. So, we have a leap year every four years to keep our calendar in sync with Earth’s orbit.

How does Earth’s elliptical orbit affect our distance from the sun?

Earth’s elliptical orbit means its distance from the sun varies throughout the year. We are closest to the sun (perihelion) around early January and farthest (aphelion) around early July, but this difference in distance doesn’t significantly affect our planet’s climate.

In this post, we’ve reviewed how Earth’s rotation and its orbit around the sun shape our world. The rotation of Earth creates the cycle of day and night, setting a natural rhythm for all life forms. Even though we don’t feel it, this rotation influences our weather and the environment every single day.

Then there’s Earth’s journey around the sun, which, combined with its tilt, gives us the seasons. This revolution impacts everything from the weather we experience to the crops farmers grow.

Earth’s spinning on its axis and its path around the sun do more than just mark time; they define our days, bring about the seasons, and shape the world we call home.

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Geography Notes

Essay on the earth: top 8 essays on earth.

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Here is a compilation of essays on ‘Earth’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Earth’ especially written for school and college students.

Essay on Earth

Essay Contents:

• Essay on the Energy Intercepted by the Earth

Essay # 1. Origin of the Earth :

The earth came into existence between 5000 to 6000 million years ago from condensed form of a cloud of gases. By studying the hot, luminous gases of the sun, we find that sun is made of the same basic elements that are found by chemical analysis of earth’s material. In fact all the stars which have been studied seem to have the same elements.

Origin of Continents and Basins :

At present the crust of the earth is largely made up of two different kinds of materials of rocks called granite and basalt. The average specific gravity of the earth is 5.5 while that of granite and basalt is 2.7 and 3.0, respectively. Granite is typically found in continental areas and the ocean floors are made of basalt.

If rafts of basalt and granite could be imagined as floating on a very heavy plastic material, the elevation of continents could be imagined as being due to the lower specific gravity of granite in the continents and the greater specific gravity in the basins.

We do not know just how the surface of the crust became separate into granite and basaltic sectors. One theory is that while the earth was still liquid, masses of granite, like flocks of foam, floated in a still liquid basaltic sea.

When the crust was solidified, the granite masses projected to form the continents. Another hypothesis holds that the continents have been growing throughout earth’s history by building of successive thick mountain ranges.

Essay # 2. Composition of the Earth :

The outer envelopes of the gaseous material surrounding the earth are called atmosphere. Under the atmosphere is our earth on which we live. That part of the earth, which is in the form of a land, is known as the earth’s crust. It also includes the highest peaks of mountains and floors of the oceans. Part of the land, which is visible on the Globe, is called the Lithosphere (Greek, Litho = Stone).

We know that nearly 75 per cent of the whole surface of the earth is covered with natural waters like oceans, seas, lakes, rivers etc. Which is in the form of more or less, a continuous envelope around the earth.

This envelope of water is called Hydrosphere (Greek, Hudous = Water). Thus, Lithosphere and Hydrosphere in a combined form is known as the Earth’s crust. Under the Earth’s crust is the interior of the Earth. It is further sub-divided into three shells. Depending upon the nature, the material is made up as shown in the Fig. 1.1.

The earth is composed of different rocks. In an ordinary sense the term rock means something hard and resistant but the meaning of the word has been extended so as to include all natural substances of the Earth’s crust, which may be hard like granite or soft like clay and sand.

It has been estimated that 95 per cent of the Earth’s crust is made up of primary i.e., first formed (Igneous) rocks which is mostly composed of Granite having Quartz, Feldspar, Biotite mica and Hornblende in varying proportions the remaining 5 per cent of the crust is made up of Secondary (Sedimentary or Metamorphic) rocks (as shown in Fig. 1.2). The Earth’s crust is in the form of a very thin layer of solidified rocks and is heterogeneous in nature.

These rocks may be classified on the basis of their density into the following two groups:

1. Sial (Si = Silicon and A1 = Aluminium) having density 2.75 to 2.90.

2. Sima (Si = Silicon and Ma= Magnesium) having density 2.90 to 4.75.

It has been estimated that the Sial rocks are about 70 per cent of the Earth’s crust, which include chiefly Granite and Silica. These rocks are generally on the upper regions of the crust.

Sima rocks include heavy and dark coloured rocks like Basalts. In these rocks, the percentage of Silica is reduced and Magnesium attains the next importance in place of Aluminium of Sial rocks. These rocks are generally found on the floors of the Oceans and beneath Sial rocks.

It is the part of the earth below the crust and surrounding the core. The imaginary line that separates the lithosphere from the mantle is known as ‘Moho’ (Mohorovicic discontinuity). Because of high temperature and great pressure, the mineral matter in this part is the molten condition.

It is the innermost layer of the earth; it extends from below the mantle (Gutenberg discontinuity) to the central part of the earth.

On the basis of earthquake waves, the core has been further divided into two cores:

(a) Outer core.

(b) Inner core.

The outer core is 2,250 km thick and surrounds the core. It is believed that it is still in molten condition.

The inner core is also called ‘Nife’ because it consists of Nickel and iron. Its thickness is about 1,228 km. It is very hard in nature.

Essay # 3. Motions of the Earth :

The earth is held in space by combined gravitational attraction of sun and other heavenly bodies and has motions that are controlled by them.

The two principal motions of earth are:

1. Rotation of earth about its axis

2. Revolution of earth around sun

(i) Rotation :

The earth rotates upon an imaginary axis, which owing to the polar flattening is the shortest diameter. The earth rotates from west to east (in anti-clockwise direction) and it takes 24 hours to complete one rotation. During this period most of the places on the sphere are turned alternately towards and away from the sun, have experienced a period of light and darkness.

This causes day and night. This unit of time is called solar day. The direction of rotation (west to east) not only determines the direction in which the sun and stars rise but is also responsible for the direction of prevailing winds and ocean currents.

Importance of earth’s rotation :

The effects of the earth’s rotation are of great importance to the environment. The rotation indirectly accounts for the diurnal changes in weather such as warming up during daytime and cooling down at night. Thus the rotation affects diurnal rhythm, day light, air temperature, air humidity and air motion. Plants and animals respond to this diurnal rhythm. Green plants store energy during day and consume some part of heat during night.

Rotation of earth turns both air and water in one direction. The flow of air and water are turned towards right in the northern hemisphere towards left in the southern hemisphere. This phenomenon is called the coriolis effect. It is of great importance in studying the earth’s systems of winds and ocean currents.

(ii) Revolution :

The path of the earth around the sun is called orbit and the rotating earth revolves in a slightly elliptical orbit about the sun from which it keeps an average distance of 150 million km.

The time required for the earth to pass one complete orbit fixes the length of the year and this journey takes a few minutes less than 365 1/4 days (365.242 solar days). Earth revolves around the sun in anticlockwise direction. The rate of earth’s revolution is more than 1,06,260 km/hour.

Importance of revolution :

The rotation and revolution of earth are of great significance in meteorology. The rotation indirectly accounts for the diurnal changes in weather such as warming up during daytime and cooling down at night. Seasonal changes are dependent on the revolution of earth. When the earth is at perihelion (January 3), the sun is close to the earth, as a result greater intensity of solar radiation is received at the earth surface.

This position occurs during winter season. When the earth is at aphelion, the earth is farthest from sun, as a result the heat received at the earth surface is less. This occurs during summer season (July 4). However, the distance between sun and earth varies only about 3 per cent during one revolution.

As the earth moves forward in its orbit, its axis remains inclined at 23 1/2° from the perpendicular to the plane of the earth’s orbit. This tilt of 23 1/2° does not change throughout the year as the earth revolves around the sun.

It causes the change in seasons regularly through spring, summer, autumn and winter because of the inclination of the earth’s axis, constant direction of tilt of that axis and revolution of the earth around the sun.

Sidereal day :

The true rotation time is called sidereal day. It is denoted by ‘S’.

The number of hours, minutes and seconds in a sidereal day are given below:

Lengths of the day :

Earth receives solar radiation from the sun during day time. Day length can be defined as the total time between sunrise and sunset. Length of the day is partly controlled by the latitude of the earth and partly by the season of the year. The day length at the equator is about. 12 hours throughout the year, whereas at the poles it varies between 0 and 21 hours from winter to summer.

Solar radiation received at any location of the earth depends upon the day length. Maximum amount of solar radiation is received in the higher latitude during summer solstice because it is period of continuous day.

The amount of solar radiation received during the December solstice in southern hemisphere is theoretically greater than that received in the northern hemisphere during the June solstice. The equator has two radiation maxima at the equinoxes and two minima at the solstices.

Length of the day plays an important role in the life cycle of the crop plants. In fact, day length indicates the photoperiod available for the growth of the crop plants. Every plant requires different photoperiod for the initiation of flowering. On the basis of day length, plants can be divided into different categories. The plants which require less than 10 hours day length, are called short day plants.

If the requirement of the plants is greater than 14 hours day length, then these are called long day plants. In between these two types, there are intermediate plants, which require photoperiod of 12 – 14 hours. However, the plants which are not affected by day length, are called day neutral plants.

Essay # 4. Movement of the Earth:

Many changes on the crust of the earth can be seen as a result of the works of internal forces in the earth’s interior. The works of internal forces are generally called earth movement. Sometimes, the earth movement may be very very slow and sometimes it may be sudden.

It is believed that originally the landmasses were united together in the form of a great landmass known as Pangaea. In course of time the Pangaea had broken into several pieces and drifted into different directions. The drifting is called Continental Drift and the theory was propounded by Alfred Wagner. The northern part of the landmass was known as Laurasia.

Eventually it had broken down to form North America, Europe and Asia. About 120 million years ago, the southern part of India, East Africa, Madagascar, Australia, South Africa, South America and the Antarctica were together and formed the single landmass known as ‘Gondwana land’. The ‘Gondwana land’ started breaking into several pieces and India took its present shape about 60 million years ago.

During the last million years, the Himalayas had risen to its present height due to earth movements. Similarly, it has been proved that the Aravallies and the Vindhyas in the middle of India were once at the bottom of the sea.

The forested areas near Bombay harbour, the Mahabalipuram temple in the sea, submergence of a vast area of nearly 5000 sq km in the Runn of Kutch during 1819 and a land of about 1500 sq km raised to a height of several metres, are some of the results of the earth movement.

The earth movements which bring about vast changes are called Tectonic movements. It has been already mentioned that earth movements may be very slow and sudden.

Slow Movements :

The slow movements of the earth’s crust are due to various chemical and physical reactions that take place at the earth’s interior. The movement may be so slow that its result may not be seen on the surface during 100 to 200 years.

The raising of the eastern coastal plain up to a height of 15-30 metres, the existence of coal beds below the sea level in the Sundarban Delta, the existence of a forest near the Bombay harbour and submergence of a vast area in the Runn of Kutch are some of the Indian examples of slow earth movement.

A change in sea level in its advance or retreat with respect to adjacent land is relative to each other. When the sea advances to land, it is generally called a Positive Movement and the land advancing against the sea is known as Negative Movement.

On the basis of the structural changes that are caused by the tectonic movement, the earth movements may be grouped into two classes:

(i) Vertical or Epeirogenic Movement

(ii) Horizontal or Orogenic Movement.

Vertical Movement :

Due to earth movement some parts of earth surface may be raised or sunk with respect to the surrounding areas. This type of movement is known as Vertical Movement. When a part of the earth’s crust is raised in relation to its surrounding area, it is known as uplift. In the same way when a portion is sunk in relation to its surrounding areas, it is called subsidence.

Earth movement of this type, when takes place over extensive area generally leads to the building up of continents and Plateaus. That is why, this type of movement is also known as Epeirogenic movement or Continent building movement. As a result of their movement, the horizontal arrangement of the earth’s crust remains almost undisturbed.

Millions of years ago there used to be a continent where we find Atlantic Ocean today. In the beginning of the earth’s history such movements had been more frequent and the present-day arrangement of the continents might have been the result of this movement.

Horizontal Movement :

The forces of horizontal movement affect tangentially. It involves both the forces of compression and tension. These two types of movements are related to each other. Compression in one part of the crust is bound to produce tension at another place. The compression leads to the bending of horizontal layers into a shape known as fold.

The tension is responsible for breaking of rock layers with subsequent sliding or displacement. It is known as fault. The processes of making folds and faults are known as folding and faulting.

When two horizontal forces act towards a common point from opposite directions folding takes place. Deep within the earth, this force tends to cause bending of rock strata. Like seawaves, rocks are thrown into upfolds and downfolds. The upfolds are called anticlines and the downfolds are known as synclines.

When two forces act horizontally in opposite directions from a common point, it generates tension and the process is known as faulting. As a result, the rocks break along a line which is known as fault line. The faulted rocks may be thrown upwards or slided downwards.

The mountains over the surface of the earth owe their origin to the process of folding and faulting. That is why, the horizontal movement is also known as orogenic or mountain building movement. The Himalayas on the northern border of India, the Alps of Europe, the Rockies of the North America and the Andes of South America are some of the newly folded mountain ranges of the world.

Aravallies, Ural, Tiensan and Appalachia are some of the old folded mountains of the world. Similarly, the Black Forest of Germany, the Voges of France, the Vindhyas and the Satpura of India are some of the examples of fault or block mountains of the world.

Plate Tectonics :

Plate tectonics is the most modern theory about the formation of folded mountains. According to this theory, the world has been divided into six major plates and several smaller plates. Each of the plates is composed of crust up to a depth of 100 km from the surface of the earth.

Due to the forces at the earth’s interior, these plates are moving in different directions. As a result of rubbing of the two plates, the folded mountains have been formed at the edges of the plates.

The six major plates are:

(1) Pacific Plate

(2) North American Plate

(3) South American Plate

(4) African Plate

(5) Eurasian Plate

(6) Indo-Australian Plate.

The smaller plates are:

(a) China Plate

(d) Nazca Plate

(c) Cocos Plate

(b) Antarctic Plate

(e) Caribbean Plate

Where plates separate and new ocean floor is created, mid-ocean ridges are the boundaries. The plates are rigid. Their boundaries are marked by earthquakes and often by volcanoes. Where plates collide and overlap, young mountains, arcs and trenches are the boundaries.

Sudden Movement :

Sudden movement of the earth crust can be noticed during earthquake. Some parts of the land surface of New Zealand were raised by about 3 metres during the earthquake of 1885. Similarly, some areas of Japan sank by about 6 metres during 1891 earthquake. Recently, during the earthquake of 1950, the bed of the Brahmaputra River had been raised leading to various changes in the valley.

Essay # 5. Interior of the Earth :

To know exactly about the interior of the earth is more difficult than that of taking photograph of other planets with the help of satellites or by walking on the surface of the moon. It has not been possible till today to collect direct evidences about the structure of the earth.

However, geographers and geologists have collected indirect evidences about the structures and composition. On the average, the radius from the centre to the surface of the earth is 6320 km but, the deepest mine in the world in South Africa is about only 4 km deep and man could dig up to a maximum depth of 6 km in search of oil.

In other words, man has been able to get direct evidences about the structure and composition of the earth’s interior up-to a depth not more than 5 to 6 km from the surface.

The knowledge beyond this limit is based primarily on indirect scientific evidences. The indirect evidences are based on temperature and pressure inside the earth, density of materials and behaviour of earthquake waves. Still uncertainty persists. On the basis of the different scientific observations it has been concluded that there exists different layers inside the earth.

Temperature and Pressure of the Earth’s Interior :

The hot and molten lava, ash, smoke that come out at the time of volcanic eruption as well as the hot water springs are some of the evidences which confirm that interior of the earth is having a very high temperature. It has been found from mining operations also that the temperature increases at the average rate of 1°C per 32 metres depth.

At this rate of increase of temperature, the rocks at great depth of the earth’s interior should be in molten state. Actually, this was the view earlier that the crust of the earth is floating on a massive molten materials.

But, the study of earthquake waves has indicated that the temperature does not increase uniformly from the surface to the centre of the earth. The rate of increase in temperature is not uniform. Scientists also have proved that the main reasons of increase in temperature are the fusion of radio-active materials and other chemical reactions. The tremendous pressure from the overlaying materials makes the melting point higher.

On the basis of this, in upper 100 km, increase of temperature is estimated to be 12°C per km; in the next 300 km it is 2°C per km and below it 1°C per km. At this rate the temperature at the core of the earth is estimated to be 6000°C.

At this temperature, the materials in the central part of the earth’s interior should have been at gaseous state but due to tremendous pressure from the outer layers, the materials assume liquid properties and acquire properties of solid or plastic state. Therefore, the earth behaves mostly as solid down to a depth of 2900 km because of tremendous pressure.

Density and Composition of the Earth’s Interior :

By studying the speed and path of earthquake waves, temperature and pressure conditions inside the earth, scientists are of the opinion that the physical properties, density and composition of the materials are different at varying depths. The structure of the earth is therefore layered. The earth consists of three layers, one inside the other like an onion. They are the crust, the mantle and the core.

The topmost layer of the earth is solid, the thinnest, and the lightest and is known as Lithosphere. The lithosphere has again two layers-outer part immediately below the newer sedimentary formation, popularly known as crust and the inner part of greater strength. The crust of the earth is composed of sedimentary and granitic rocks.

The inner layer of lithosphere has basaltic and ultra-basic rocks. While the outer layer of lithosphere is found mainly under continents, the inner layer is found partly under oceans. The average density of lithosphere is 2.65 to 2.90. ‘Silica’ and ‘Aluminium’ are abundant. Therefore, it is popularly known as SIAL (Silica + Aluminium). The average thickness is 8 to 100 km.

Below this top layer is the layer of basalt rock which is heavier than the topmost layer. The density varies from 3.1 to 5.00. It assumes the properties of solid and partly plastic materials. The average thickness of this layer is 100 to 2900 km. In this layer, Silica and Magnesium elements predominate and it is popularly known as SIMA (Silica + Magnesium).

The SIMA also has two layers—Inner silicate layer at the top with average thickness of 100 to 1700 km and Transitional zone of mixed metals and silicates with an average thickness of 1700 to 2900 km. These two SIMA layers are also known as MANTLE. The surface that separates crust and mantle is known as Mohorovicic Discontinuity or simply MOHO.

Finally, the innermost layer exists at the central core of the earth with density 5.1 to 13.00. It is composed of the heaviest mineral materials. This central mass is mainly made of ‘nickel’ and ‘iron’ therefore known as NIFE (Nickel + Ferrous). The core contains 1/3 of the entire mass of the earth.

The materials of this part may be in liquid, plastic or even solid state due to tremendous pressure from above. It has also two layers-outer metallic core with average thickness of 2900 to 4980 km and inner metallic core between 4980 to 6400 km. The inner metallic core is also known as Barysphere (Fig. 2.4).

The three layers of the earth have been called by different geologists in different manners. German scientist Gracht called them SIAL, SIMA and NIFE. Jeffrey called them as Top, Middle and Lower layers while Professor Holmes called them the Crust, the Substratum and the Core. The relationship has been mentioned in Table 2.2.

From Table 2.2, it can be estimated that crust of the earth forms less than 1 per cent; mantle 16 per cent and 83 per cent makes the core. The earth being a spherical body has materials of varying densities at varying depths.

Materials of the Earth’s Crust :

The word ‘Lithosphere’ means a sphere of rocks. The upper portion of lithosphere is referred to as the crust of the earth. Down to depth of nearly 16 km from the surface under the continents, 95 per cent of the materials that form the crust consist of rocks and the rest 5 per cent minerals.

The term rock refers to hard masses of earth’s crust as well as loose and soft particles like sand and clay. The rocks are formed of the mixture of various minerals. All rocks do not have same chemical composition and structure.

But, every mineral has its own chemical composition and physical properties. The minerals generally occur in the form of crystals. The rocks and minerals are generally composed of certain chemical elements like oxygen, silica, aluminium, iron and calcium, etc.

Each mineral usually contains two or more simple substances called elements. There are about 2000 minerals but only 12 are common all over the earth. These 12 minerals are basically responsible for the formation of rocks.

Mineral may be defined as a naturally occurring non-living solid substance possessing certain physical properties and definite chemical composition. The minerals may be either elements or compounds and also metallic or non-metallic.

The most abundant elements in nature are silicates, carbonates, chlorides, sulphates and oxides. As much as 87 per cent of the minerals of the earth’s crust are silicates and 59 per cent of the rocks are formed of the minerals of silica group.

The distribution of minerals in the earth’s crust is as follows (Table 2.3):

Minerals are of two types -Rock forming and Ore forming:

It is one of the most abundantly available minerals in earth’s crust. It has two elements— Silicon and Oxygen. They unite together to form a compound, known as carbonate of lime. It is transparent in its pure state. However, quartz may be of different colours when it is mixed up with other elements. Its hardness is 7, specific gravity 2.65. Quartz is hexagonal and its structure is SiO 2 (Silicon dioxide).

Feldspar is one of the important elements of rock and nearly 50 per cent of the earth’s crust is composed of feldspar. It is made of silicates of aluminium, potassium, sodium, calcium and oxygen. Feldspar is of two types—Orthoclase and Plagioclase.

Orthoclase has specific gravity of 2.57, its hardness is 6 and structure is Ca 2 SiO 4 (Calcium Silicate). The specific gravity of plagioclase feldspar is 2.60 to 2.74, hardness is 6 to 6.5 and the structural formula is Na 2 OAl 2 O 6 SiO 2 (Sodium Aluminium Silicate) and CaOAl 2 O 3 2SiO 2 (Calcium Aluminium Silicate).

Mica is formed of the elements of hydrogen, potassium, aluminium, magnesium, iron and silicon. Mica is of two types—Black Biotite and White Muscovite. Mica is found in thin sheets.

Its hardness is 2.5 to 3, specific gravity is 2.70 to 3 and the structural formulae are:

Black Biotite – (AlFe) 2 (MgFe) (HK) 2 (SiO 4 ) 3

White Muscovite – K 2 O 3 Al 2 O 3 6SiO 2 2H 2 O. (Potassium Aluminium Silicate)

Calcite is formed of the chemical composition of calcium, magnesium, carbon dioxide and oxygen. It is white in colour, it may take other colour also. Its hardness is 3, specific gravity 2.70 and structural formula is CaCO 3 (Calcium Carbonate).

Magnetite :

It is composed of Silicon, Iron and Oxygen. Its hardness is from 5.5 to 6.5 and specific gravity is 5.19. Magnetite is not transparent and chemical formula is Fe 3 O 4 (Ferros Ferric Oxide).

Haematite :

Haematite is also made up of Iron and Oxygen. Its hardness is 5.5 to 6.5, specific gravity is 4.9 to 5.3 and structural formula is Fe 2 O 3 (Ferric Oxide).

Graphite is another mineral made of carbon. Its hardness is 1.5 to 2, specific gravity is 2.15 and structural formula is C (Carbon).

In addition to the above, there are many other rock and ore forming minerals. The minerals that form with oxygen are called oxides. Quartz, Magnetite, Limonite (2Fe 2 O 3 ) 3 , (Ferric Oxide), Cromite (FeOCr 2 O 3 ), Alumina (Al 2 O 3 ) belong to oxide group.

The minerals that form of Calcium, Carbon and Oxygen are called carbonates. Calcite (CaCO 3 ) (Calcium Carbonate), Dolomite (CaMg) (Calcium Magnesium), Cidarite (FeCO 3 ) (Ferrous Carbonate) ‘etc.’ belong to this group. Mineral salt (NaCl) belong to chloride group and Gypsum (CaSO 4 2H 2 O) (Hydrated Calcium Sulphate) are of sulphate group.

The minerals that have only one element are known as Native Minerals, Gold, Silver, Lead, Copper, etc., belong to this group.

The Rocks :

In terms of origin, the rocks can be classified into two main varieties, namely, Igneous rocks and Sedimentary rocks. But when these rocks are subjected to prolonged fluctuations of temperature and pressure, they are transformed to a new variety which is termed as metamorphic rocks.

Essay # 6. Theories of the Earth:

There are several hypotheses about the origin of the universe and the earth. In 1755, German philosopher Immanuel Kant put forward a theory that a spherical mass of gas called Nebula was rotating and its size was like that of the sun.

Due to rotation and cooling through radiation, the outer portion became denser and rings were thrown out. In course of time, these rings condensed into planets while the remnant continues as the sun (Table 2.1).

In 1796, Laplace- a great French Mathematician supported the Nebular hypothesis. However, there are several criticisms against Laplace’s theory and the most important one is that rings cannot condense into planets.

Kelvin’s Nuclear clots Hypothesis and Chamberlin and Moulton’s Planetesimal Hypothesis are other two hypotheses relating to the origin of the earth. However, they also lack in certain aspects of acceptability.

Tidal Hypothesis :

Looking from different angles, the Tidal Hypothesis of Jean and Jeffreys – well-known scientist of England has been found to be more acceptable. According to this hypothesis, the sun was a big and extensive mass of gas moving in space. Once, another much larger star happened to come closer to the sun and due to gravitational pull of this star a tide occurred on the surface of the sun.

As a result, protuberances of material from the sun came out towards the approaching star and in course of time it gave birth to earth and other planets of the solar system (Fig. 2.1).

There are several points in favour of Jean and Jeffreys (Tidal Hypothesis):

(1) If the density of the sun increases from its surface towards the interior, it is quite natural that the protuberances come out from the surface like a filament having lesser density. Naturally, the protuberances produced by the passing star should have been thicker in the middle and thinner at both the ends. The arrangement of the planets in the solar system is also like a cigar-middle portion bigger and tapering towards the two ends.

When the passing star had gone far away, the filament has been broken into pieces and due to the gravitational force of the sun started rotating around the sun. The planets of bigger size in the middle and smaller size towards the two ends can be seen in Fig. 2.2. Mercury is the smallest and nearest to the sun, Jupiter, the largest is in the middle.

(2) The arrangement of the satellites of respective planet also confirms the validity of this hypothesis. Saturn, the second largest planet has the largest number of 18 satellites and Jupiter, the largest planet has 16 satellites. Uranus and Neptune have 17 and 8 satellites respectively.

(3) Lastly, on the basis of this hypothesis it can be seen that bigger planets remained in the gaseous state for a longer time and have helped in formation of more number of satellites than that of the smaller planets which condensed quickly and did not have scope for formation of satellites.

This hypothesis is also known as Hit and Run Hypothesis or Catastrophic Hypothesis or Tidal Action Hypothesis.

Tidal Disruption Theory :

The earth and planets and their satellites were all part of the sun or another sun like star at that time. There are many theories regarding the formation of the solar system and our earth. One is the tidal disruption theory by Jaans and Jeffereys. This theory states that in the beginning sun was hot and in a gaseous state.

A big star moved across, which caused the tidal disruption of hot gas and it left the sun with a revolving arm or a filament of hot gas, like a spiral nebula .The cooling of filament broke up the sun into masses which began to contract toward nuclei forming planets. The gaseous planets and their satellites continued to revolve as they did after the star passed away. Gradual cooling formed liquids and final solids.

The more volatile material of the earth remained in the gaseous state and formed our atmosphere, originally much deeper and of a higher temperature than now. As the atmosphere cooled, the water vapour condensed and formed clouds. As cooling continued, rain fell and oceans formed.

Steady State Theory:

Another theory which can be considered as alternative to the Big Bang Theory is the Steady State Theory. It is propounded by Hoyle. Here, Hoyle propounded that the Universe remained of the same size at any given point of time. However, this theory has been discarded after evidences of expanding universe.

Big Bang Theory :

Another recent and most convincing proposition regarding the origin of the universe including the planet earth is the Big Bang Theory or Expanding Universe Theory. Eduin Hubble provided evidence of expanding universe in 1920. The theory assumes that the universe began from huge mass of atoms called primeval atoms or cosmic eggs having a state of infinite density.

The universe initiated its origin 15 million years ago when a dense mass of material exploded in the so-called big bang. The explosion sent all of the materials of the universe outward in a cosmos that is still expanding. All of the galaxies, planets, asteroids, and other bodies in the universe were formed from the gas and dust of this extraordinary explosion.

This theory was put forward by an astronomer cum priest named George Lamaitre in 1927. It is felt that the expansion of the universe will continue for a long time. After that perhaps the expansion will slow down. At any point of time, if similar condition prevails the stimeval atoms may become active again and another explosion (big bang) may take place.

Although many scientists contributed to the development of this theory, it was George Gamow who coined the term Big Bang in 1946. Gamow with RA Alpher envisaged a high temperature state in the beginning of the universe.

Evolution of the Earth and Life forms :

The evolution of the earth can be described as follows in terms of various stages. In the initial stages the earth was barren, rocky and hot. It had thin layers of hydrogen and helium. During the period of 4000 million years till now the earth had been through several processes and life evolved.

From the top of the atmosphere to the centre of the earth different density materials have formed different layers. The process of separating denser materials from the lighter materials is called differentiation. The present atmosphere contains water vapour, nitrogen, carbon dioxide, methane and small quantity of oxygen. Plants are the major sources of oxygen on earth.

The oceans formed within 500 million years from the formation of the earth. Life began approximately 3000 million years ago. The process of photosynthesis began between 2500 and 3000 million years ago. The first life of the earth was confined to oceans in the form of small bacteria. The evolution of life from bacteria to modern man is shown by Geological Time Scale expressed in terms of Eons, Era, Period and Epoch.

Essay # 7. Numerical Facts about the Earth :

The earth is spherical in shape with a bulge at the middle and a slight flattening at the poles. The equatorial radius is 6374 km and the polar radius is 6357 km. The mean distance from the sun is 150 million km.

Continental Drift Theory :

The making of the continents began 200 million years ago (during the Persian period) with the split of gigantic landmass known as PANGEA. Two continents laurasia to the north and Gondwana to the south were formed. Later on, these were sub divided into smaller parts approximately the shapes of Africa, Eurasia North and South America, Australia and Antarctica as we know today. This theory is known as continental drift theory.

Continents and Oceans of the World :

During Persian period, outer surface of the earth was broken into 10 major and a number of minor sections called plates. It is on these plates that continents rest. The rifts between the plates were filled with molten material from the mantle pushing the plates to either side and farther and farther as the material continued to seep through. Since this material was heavier, it levelled off below sea level forming ocean floors as water from the pacific flowed in.

A continent is a large, continuous area of land on the earth. All continents together constitute less than one-third of the earth’s surface, more than two-third of the earth’s surface are covered with water. Two third of the continental land mass is located in the northern hemisphere.

There is no standard definition for the number of continents in the world. By most standards, there are a maximum of seven continents Africa, Antarctica, Asia, Australia/Oceania, Europe, north America and south America. In Europe, many students are taught about six continents, where north and south America are considered to form one America.

Many geographers and scientists now refer to six continents, where Europe and Asia are combined, called Eurasia, because they are one solid land mass. By the definition of a continent as a large continuous area of land, the pacific islands of Oceania are not a continent, but one could say, they belong to a continent e.g. Oceania is sometimes associated with the continent of Australia.

Essay # 8. Energy Intercepted by the Earth :

Radius of the earth = r

Area of the earth = π r 2

Solar constant = S

Total energy intercepted by the earth in unit time = π r 2 S

= 6.37 x 10 21 cal day -1

Surface area of the earth = 4 π r 2

If this energy is spread uniformly over the full surface of earth, then the energy received per unit area per unit time (Q S ) can be given as follows:

But the distribution of solar radiation over the earth surface is not uniform, annual value at the equator is 2.4 times that at the poles. Incident solar energy at the surface depends upon geographic location, orientation of the surface, time of the day, time of the year and atmospheric conditions i.e. clear, cloudy, foggy etc..

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The Effect of the Earth’s Rotation & Revolution

• The Effect of the Earth’s…

The Effect of the Earth’s Rotation & Revolution

When watching the stars at night, they do appear to move very slowly.  This is because the Earth is constantly moving. The Earth completes one “rotation” every twenty-four hours.  A rotation is when the planet spins around once. The Earth rotates counterclockwise; this is why the Sun “rises” in the East and “sets” in the West.  It is not the Sun’s movement that causes days, but rather the Earth turning around in front of the Sun.

The Earth’s axis (the point at which it rotates around, for example, if you were to spin around while standing in one spot, your axis would be an imaginary line running through your head straight down to your feet) is in line with a star named “Polaris”. Polaris is also known as the “North Star” since it is directly above the Earth’s axis. 

Since this star is directly above the Earth’s axis, it does not appear to move, however, the rest of the stars in the sky move around Polaris (for example: when you spin around, the object directly above your head does not appear to move but everything else seems to spin around that object). Polaris is only seen in the Northern hemisphere and it belongs to the Little Dipper constellation (it’s the last star at the end of the “handle”).

The Effect of The Earth’s Rotation

Another type of motion is known as “revolution”.  Revolution is when one object completes a circular path around another object. The Earth takes 365.24 days to revolve around the Sun. This is why a year is 365 days long. During the year the Earth is angled differently towards the Sun.  These changing angles provide us with different Sun intensities and therefore we get four different seasons. Since the Earth is at different positions in space over the year, we see different constellations throughout the year.

Coriolis Effect: Defection of wind due to rotation of Earth

UP [NORTH]: West DOWN [SOUTH]: East (On Surface)

Northern Hemisphere: Deflected to the right (clockwise)

Southern Hemisphere: Deflected to the left (counter-clockwise)

Trade Winds: high pressure wind blown to the west from 30N

Westerlies: deflected to the east

Earth is currently in a cool phase characterized by formation of glaciers ( glacial maxima ), followed by warm periods with glacial melting ( interglacial periods ). These glacial–interglacial cycles occur at frequencies of about 100,000 years. We are currently in an interglacial period; these have lasted about 23,000 years in the past. The last glacial maximum was about 18,000 years ago.

The glacial–interglacial cycles have been explained by regular changes in the shape of Earth’s orbit and the tilt of its axis— Milankovitch cycles .

Circular rotation causes glaciers to melt; more solar radiation; Elliptical= less radiation. The intensity of solar radiation reaching Earth changes, resulting in climatic change. The shape of Earth’s orbit changes in 100,000-year cycles. The angle of axis tilt changes in cycles of about 41,000 years. Earth’s orientation relative to other celestial objects changes in cycles of about 22,000 years.

The Effect of Planet’s Motions

Thousands of years ago, people were able to clearly see the night sky (no “light pollution”).  The one thing they noticed is that five “stars” seemed to wander faster through the night sky than other stars.  These “stars” were actually the planets Mercury, Venus, Mars, Jupiter, and Saturn. People called these objects “wandering stars”. 

Their names were then changed to planets which is after the Greek word “planetes” which means “wanderers”. All planets rotate on their axes and revolve around the Sun, however these times are different for each planet. Planets move through constellations as well.  This motion usually takes a few weeks. Many constellations are named after animals. 

The Greek word for “animal sign” is “zodion”.  This is why we have star groups called the zodiac constellations. Depending on which zodiac constellation was visible when you were born is the “sign” you have been assigned.  For example: Aquarius, Leo, Gemini, Sagittarius, etc. Many people believe that zodiac signs determine certain traits and characteristics of people.  This is known as “astrology” and is not a legitimate science based on truth or facts.  Astrology is simply for entertainment.

Revolution Around the Sun vs. Rotation upon Axis

Revolve, as in orbiting the Sun? Yes, all the planets in our solar system orbit the Sun in the same direction Earth does. Some comets and asteroids orbit backwards, and some (more so comets than asteroids) orbit virtually perpendicular to the plane of Earth’s orbit.

Rotate, as to spin on ones axis (the thing that causes day and night on Earth)? Earth rotates counter-clockwise, as seen from above Earth’s north pole, the same direction it revolves around the Sun. But two planets (used to be 3, when Pluto was a planet) rotate clockwise – Venus and Uranus. Some might quibble about Uranus, as it spins on its side, but technically it rotates clockwise.

Why do they all revolve in the same direction, and most rotate in the same direction? Because of the way the solar system formed. It formed out of a nebula – a giant cloud of gas and dust in space. This cloud had a slight rotation to it. Gravity caused the dust and gas to come together, but since the nebula was spinning, it collapsed into a disk instead of a sphere.

The center of the disk, that’s where the Sun formed. The rest of the disk (now rotating quite nicely) is where the planets formed. So all the planets revolve in the same direction because that’s the direction the original nebula was rotating.

Why do some planets now rotate backward? They got clobbered by one or more large asteroids while they were forming, which caused their rotation rate/direction to change. Earth got clobbered, too, at least once – that’s how we got our Moon!

Effects of Earth’s revolution and tilt

The Earth’s revolution has several effects including the seasons and the variable duration of days/ nights. Also, the Earth’s tilt and axis relative to its orbital plane have a significant effect as well. This results in one hemisphere tilting toward the sun and the contralateral hemisphere tilting away. The hemisphere tilted towards the sun will experience warmer weather and longer daytime hours.

Whereas the hemisphere titled away from the sun will experience cooler temperatures and shorter daytime hours. This variation in daytime hours and average temperature cases by revolution and tilt results in the different seasons of the year. If the Earth were exactly perpendicular to its orbital plane, the seasons would not occur. It would also cause both hemispheres to experience approximately 12 hours of daylight and darkness during a 24-hour period. The Earth’s current axis is 23.5 degrees, if it were to be tilted more, this would result in warmer summers and colder winters. respectively.

For example, the summer solstice occurs when the Northern Hemisphere is at its maximum tilt toward the sun. During this period the sun will be directly overhead long the latitude of 23.5 degree N; otherwise known as the Tropic of Cancer. During the first day of summer, location along the latitude of 23.5 degree of the North pole experience 24hrs of daylight.

Altitude & Latitude

First, altitude describes how high a certain point is located above sea level. It mainly affects the climate in regions situated at high altitudes by making them cooler as the air pressure and temperature decreases. An example of a high-altitude region is the Himalayas, with an altitude of nearly 9000 meters, and fall in temperature from 0.2 to 1.2 degree Celsius every 100 meters.

These regions are typically characterized by high amounts of precipitation, strong winds and low levels of oxygen due to the lower air pressure. Altitude does affect climate, but primarily the local climate of a specific location; it does not contribute to affecting the entire planet’s climate. This thus downplays its significance in contributing to the Earth’s climate.

The further away a location is from the equator, the less sunlight it receives to heat the atmosphere because the sun’s rays are dispersed over a larger area of land as you move away from the equator due to the curvature of the Earth. As a result, places nearer to the equator such as the Sahara Desert tend to be hotter with a mean temperature of 30-40 degrees Celsius, as opposed to the polar regions with an average temperature of 0 to -40 degrees Celsius.

E vidence of the Earth’s Rotation and Revolution

Computing the speed of Earth’s revolution around the Sun:

•Circumference of Earth’s orbit = 940,000,000 kilometers

•Time for one revolution = 365 1/4 days = 8766 hours

•Speed of revolution = Distance/Time = 940,000,000 km / 8766 hr = 107,000 km/hr = 30 km/sec

•Uniform motion is difficult to detect. Although it is possible to detect the Earth’s circular motions, the effects are subtle, and were not detected until the 18th and 19th centuries, long after Copernicus proposed that the Earth was in motion.

The Coriolis effect was first described in 1835 by a French scientist by the name of Gustave Coriolis. If you are located at the equator, and fire a cannonball north or south, you find that the cannonball swerves to the east.

The net result of the Coriolis Effect: In the Northern Hemisphere , projectiles swerve to the right. In addition, air rushing inward to a low pressure area will swerve to the right, and set up a COUNTERCLOCKWISE hurricane. 

In the Southern Hemisphere , projectiles swerve to the left, and air rushing inward to a low pressure area will set up a CLOCKWISE hurricane.

The Foucault pendulum was first demonstrated in 1851 by yet another French scientist; Jean Foucault. The Foucault pendulum is nothing more than a very long pendulum suspended from a well-oiled ball-and-socket joint overhead, so it is free to swing in any direction. 

Foucault set up such a pendulum in the Pantheon in Paris, and set it swinging north to south. As hours passed, however, the direction in which the pendulum was swinging moved around in a clockwise direction. After a while, the pendulum was swinging northeast-southwest; after a while longer, it was swinging east-west, then southeast-northwest, then north-south again.

What causes this change in the pendulum’s direction of swing? The rotation of the Earth, of course.

The important fact (independent of where you’re standing) is that the Earth and the pendulum’s swing are rotating relative to each other. If the Earth did not rotate on its axis, the direction of swing of a Foucault pendulum would remain fixed relative to the surface of the Earth.

When Copernicus proposed his heliocentric theory, his critics pointed out that if the Earth orbits the Sun once per year, then the Earth’s location in October (for instance) should be 2 astronomical units away from its location in April, half a year later. This change in the Earth’s location must cause the nearby stars to shift in apparent location relative to more distant stars.

Stellar parallax was searched for by astronomers from antiquity onward. However, prior to the invention of the telescope, stellar parallax was not observed.

There are two possible hypotheses:

(1) There is no stellar parallax because the Earth is stationary. This is the hypothesis put forward by the supporters of the geocentric universe.

(2) Stellar parallax exists, but it is too small to be detected, because the stars are too far away. This is the hypothesis put forward by Copernicus and other supporters of the heliocentric universe.

In fact, stellar parallax was first detected by Bessel (using a telescope) in the year 1837, nearly three centuries after the death of Copernicus. In general terms, parallax can be defined as the shift in the observed position of an object, resulting from a change in the observer’s location.

So, Copernicus was right:

•The Earth does rotate about its axis.

•The Earth does revolve around the Sun.

•The stars are very distant from the Sun.

However, Copernicus wasn’t vindicated by direct observational evidence until centuries after his death!

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51 Comments

What about effects of rotation of the earth on its own axis

not enough information to complete my report

wow I learn a lot!

It doesn’t mention some important facts; such as when it is summer, winter, spring and autumn

The phenomena of revolution gives us?

This is actually amazing. The difference of Time from place to place is caused by the earth rotation.

I just want effect of earth revolution

I have a “Question”so i tried to look why does it affect oír view of the might sky

I need more explanation on solar system

I want further distinctions and details

Can I get the major effects of the earth movement

There has to be more information on the earth’s rotation, there was only one THING about it.

I need information on how earth’s rotation around the sun shapes the Earth. I also am needing information on how different forces shape the earth; Volcanoes, earthquakes, erosion, Earth’s rotation around the sun, ocean currents, tides, weather, and the water cycle. I am currently researching how these shape the earth and i will need more info.

In the picture with the earth rotating around the sun… if the earth rotates exactly 360 degrees, or 1 revolution every 24 hours (i.e. one revolution every day), should not the same identical picture of the earth be in each of the four positions for each of the four months indicated (jan, march, sep, dec)… oh, but that might might mess up the shading from the sun on each of the earth pictures… but, maybe some food for thought. That will really get those brain juices flowing. Just something to think about. So if it were noon in southern California on January 1, the sun would be directly overhead. Now fast forward exactly 180 days. I will have made exactly 180 revolutions around the earth. I will also have made a trip exactly 180 degrees around the sun. The problem is the sun is exactly 180 degrees behind me, which is now behind the other side of the earth. No one has ever explained that this model of the sun and earth don’t work, and you just figured it out. OOPs!

ok man i got you.

there are 2 things you aren’t taking into consideration. 1) after one day the earth only moves 1/365 of a way around the sun, not 1/360 2) sidereal days exist. a sidereal day is how long the earth takes to rotate once. the whole 360 degrees. a solar day is the day we use, and it’s the amount of time it takes for the sun to be in the exact same position. a sidereal day is only 4 minutes shorter than a solar day, so from day to day you can hardly tell, but if you take 4 minutes every day for the next 180 days you get 720 minutes. 720 minutes divided by 60 (60 minutes in an hour) gets you 12 hours, or half a rotation. so you’d still be facing the sun at solar noon 180 days later.

I only want about earths rotation

Wow, lots of info to be used

What about the spin of the Earth?

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Thank you sir

guys can you tell me how revolution and rotation affect the seasons on earth?

I love this site

Not much for revolution, which is what I need!!!!

Nice site. I need more diagrams

I need more of explicit info on the environmental effects of rotation

Need more info on effects of rotation

I need more information on the effects of rotation

I THOUGHT IT WAS VERY GOOD!

INFORMATION AND REASONS SHOULD BE EXTENDED.

Not full think but good. I need a big one on effects of revolution

Question: Is the total orbital distance traveled in one year the same distance in the “more elliptical” and the “less elliptical” cycle?… is the time it takes to complete 1 revolution around the sun the same in each maximum / minimum orbits? ( Just wonder if one year has always been the length of the current year)

I thought this was good to! I hope to read more of these articles

yap. wow. now i know every thing about earth

I need more effects of earth’s revolution

not enough info to complete homework

Completing summer holiday homework 🙁

There are other sources of information. I was taught to not get all my research from one source anyway.

Yes you are right

thank you helped me so much on my exam studies!

Our Solar System formed out of a gaseous dust cloud. When it condensed by gravity, it began to spin. That spin caused the system to flatten out into a pancake form. The Sun and planets formed but the spin never stopped. So all objects revolve around the Sun in the same direction.

DO ALL THE PLANETS REVOLVE AROUND THE SUN IN THE SAME DIRECTION?

yeah. thanks for the info. but i really need more explicit information. 🙂

Thank you so much!

@mirz claire

On what specifically?

I need more info!

I need more spesific effects of the revolution and rotation

i would like 2 learn more.

now,iknow why there is a night and day

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Rotation and Revolution of Earth

Rotation and Revolution of The Earth full process explained here. The Earth revolves around the Sun, for example, and the Moon revolves around the Earth.

Table of Contents

Revolution of Earth

The term “revolution of Earth” refers to Earth’s orbit around the Sun. Earth travels in an elliptical (oval-shaped) path around the Sun, completing one full orbit approximately every 365.25 days. This orbit is responsible for the changing seasons and the length of our calendar year.

What is Revolution of the Earth Depends Upon?

Here are some key points about Earth’s revolution:

Orbit Shape: Earth’s orbit is not a perfect circle; it’s an ellipse, which means that at certain times of the year, Earth is closer to the Sun (perihelion) and at other times, it’s farther away (aphelion). However, the difference in distance is relatively small and doesn’t significantly affect the climate.

Speed of Revolution: Earth travels at an average speed of about 29.78 kilometers per second (about 107,000 kilometers per hour or 67,000 miles per hour) in its orbit around the Sun.

Tilted Axis: Earth’s axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun. This tilt is responsible for the changing seasons as different parts of Earth receive varying amounts of sunlight during different times of the year.

Seasons: As Earth orbits the Sun, the tilt of its axis causes different parts of the planet to receive more or less direct sunlight. This variation in sunlight leads to the four seasons: spring, summer, autumn (fall), and winter.

Equinoxes and Solstices: There are two equinoxes and two solstices during Earth’s orbit. The vernal (spring) and autumnal (fall) equinoxes occur when day and night are approximately equal in length, marking the start of spring and autumn, respectively. The summer and winter solstices occur when one hemisphere (either the Northern or Southern) is tilted closest to the Sun, resulting in the longest or shortest day of the year, respectively.

Length of a Year: A year, as we commonly define it, is approximately 365.25 days long. To account for the extra 0.25 days, we add an extra day to the calendar every four years, creating a leap year with 366 days.

Earth’s revolution around the Sun is a fundamental astronomical event that governs our calendar and is responsible for the changing seasons, which play a crucial role in Earth’s climate and ecology.

Rotation and Revolution of the Earth

Rotation and Revolution of The Earth full process explained here. The movement of a planet around a star, or a moon around a planet, is known as orbital revolution. The Earth revolves around the Sun, for example, and the Moon revolves around the Earth.

Planets and moons follow elliptical paths around the sun. A planet’s orbital revolution takes one year.

A revolution is a round movement of the Earth around the Sun in a fixed path. The Earth rotates in an anticlockwise motion, from west to east. In one year or 365.242 days, the Earth completes one revolution around the Sun. The earth’s rotational speed is 30 km/s-1.

Earth Rotation and Revolution

Every 365.2564 mean solar days, Earth circles the Sun at a distance of around 150 million kilometres. This causes the Sun to appear to travel eastward in relation to the stars at a pace of around 1° every day, or one apparent Sun or Moon diameter every 12 hours. Because of this motion, it takes the Earth 24 hours to complete a full rotation around its axis, allowing the Sun to return to the meridian. Earth’s orbital speed is roughly 29.78 km/s (107,200 km/h; 66,600 mph), which is fast enough to travel a distance of about 12,742 km (7,918 mi) in seven minutes and 384,000 km (239,000 mi) in about 3.5 hours. Read About: Global Warming Read About: Afforestation Read About: Rainwater Harvesting Read About: Monsoon Read About: Irrigation System Read About: Ecosystem

Rotation and Revolution of the Earth around the Sun

The term “revolution” is frequently used interchangeably with “rotation.” However, revolution is referred to as an orbital revolution in many domains, such as astronomy and related subjects. It refers to the movement of one body around another, whereas rotation refers to the movement around an axis. The Moon, for example, revolves around the Earth, and the Earth, in turn, revolves around the Sun.

Rotation and Revolution

When an object is in orbit, it is said to be “revolving,” whereas a planet’s spin is said to be “rotating.” ‘A year’ is the time it takes for an item to revolve around the Sun, and one day is the time it takes for it to rotate about its axis.

The term “revolution” is used to describe Earth’s motion (or orbit) through space. Seasonal change and leap years are caused by the Earth’s rotation around the sun. This route is elliptical in shape, including points where Earth is closer to and farther from the sun.

Moreover, a year on Earth is 365 days long, according to the Gregorian calendar, with an extra day added every four years.

The time it takes for an object to revolve around the sun differs depending on the thing. The planet Mercury, for example, revolves around the sun in around 88 days, but the dwarf planet Pluto takes almost 248 years to do it.

However, the Earth’s typical solar day—its rotation period relative to the Sun—is 86,400 seconds (86,400.0025 SI seconds). Because of tidal slowdown, Earth’s solar day is currently slightly longer than it was in the nineteenth century, each day fluctuates between 0 and 2 milliseconds longer than the mean solar day.

Rotation refers to the circular movement or spinning of an object around an axis or a center point. It is a fundamental concept in physics and geometry and has various applications in different fields.

Here are some key points about rotation:

• Axis of Rotation: Every rotation has an axis, which is an imaginary line around which the object rotates. For example, the Earth rotates on its axis, which runs from the North Pole to the South Pole.
• Degrees and Radians: Rotations can be measured in degrees or radians. A complete rotation of 360 degrees is equivalent to 2π radians.
• Direction: Rotation can occur in a clockwise or counterclockwise direction, depending on the orientation of the axis and the direction of the spin.
• Angular Velocity: Angular velocity measures how quickly an object is rotating. It is usually expressed in degrees per second or radians per second.
• Rotational Inertia: Rotational inertia, also known as moment of inertia, describes an object’s resistance to changes in its rotation. Objects with larger rotational inertia require more torque to change their rotational speed.
• Applications: Rotation is fundamental in various fields, including physics, engineering, and mathematics. It is used to describe the motion of objects, such as the rotation of planets, the spinning of wheels, or the motion of gears in machinery.
• Mathematics: In mathematics, rotation matrices and quaternions are used to represent and manipulate rotations in three-dimensional space.
• Gyroscopes: Gyroscopes are devices that utilize the principles of rotation to maintain stability and orientation. They are commonly used in navigation systems, aviation, and spacecraft.
• Sports and Entertainment: Rotation is important in sports like gymnastics, figure skating, and diving, where athletes perform various rotational movements. It is also a key element in dance and acrobatics.
• Everyday Examples: Everyday examples of rotation include the rotation of the Earth, the spinning of a top, the turning of a steering wheel, and the movement of a wind turbine rotor.

Understanding rotation is crucial in many scientific and practical applications, as it helps us describe and predict the behavior of rotating objects and systems.

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What is the Earth's revolution?

The movement of a planet around a star, or a moon around a planet, is known as orbital revolution. The Earth revolves around the Sun, for example, and the Moon revolves around the Earth. Planets and moons follow elliptical paths around the sun. A planet's orbital cycle takes a year, while the Moon's revolution takes a month.

What is the significance of revolution to the Earth?

The seasons are determined by the revolution of the Earth.

What are the consequences of the Earth's revolution?

The Earth's rotating causes day to turn to night and night to day, and the Earth's whole rotation/revolution causes summer to turn to winter and vice versa. The Earth's spinning and revolution, when combined, affect wind direction, temperature, ocean currents, and precipitation, resulting in our daily weather and global climate.

How long does it take for the Earth to go through a revolution?

In 365 days, 5 hours, 59 minutes, and 16 seconds, the Earth revolves around the sun. A year is the length of time it takes for a planet to orbit the sun.

What do we name 'one Earth revolution'?

It takes 365.25 days and we call it 'one year'.

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Home » World Geography » Physical Geography of the World » Origin and Evolution of Universe Solar System » Motions of the Earth and their effects

The earth has two types of motions, namely rotation and revolution. Rotation is the movement of the earth on its axis. The movement of the earth around the sun in a fixed path or orbit is called Revolution.

• Imagine a line passing through the center of Earth that goes through both the North Pole and the South Pole. This imaginary line is called an  axis .
• Earth spins around its axis, just as a top spins around its spindle. This spinning movement is called Earth’s  rotation .

• The axis of the earth which is an imaginary line, makes an angle of 66½° with its orbital plane.
• The plane formed by the orbit is known as the orbital plane.
• The earth receives light from the sun.
• Due to the spherical shape of the earth, only half of it gets light from the sun at a time.
• The portion facing the sun experiences day while the other half away from the sun experiences night.
• The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis.
• The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.

The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to the region. The other half would remain in darkness and be freezing cold all the time. Life would not have been possible in such extreme conditions.

The earth rotates on its axis taking approximately 24 hours to complete one rotation. This has important environmental consequences.

• Rotation causes the day-night cycle which also creates a corresponding cycle of temperature and humidity creates a corresponding cycle of temperature and humidity
• Rotation requires the creation of standardized time zones. There are 24, one for each hour of the earth’s rotation.

• Tides are complicated complicated because because they are the result of both the gravity gravity of the moon and the gravity of the sun. Sometimes the sun and the moon are lined up with the earth, but most of the time they are not. Tides are highest when the earth, sun and moon are in a straight line.
• Rotation causes a deflection of ocean and air currents. The earth rotates much faster than the winds or currents move. This causes a large deflection in the direction that winds move and ultimately results in rotation around low pressure cells and high pressure cells. It also causes large rotating pools of water in the oceans called gyres.
• At the same time that the Earth spins on its axis, it also orbits, or revolves around the Sun. This movement is called revolution.

• It takes 365¼ days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.
• Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February.
• Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.
• The gravitational pull of the Sun keeps Earth and the other planets in orbit around the star. Like the other planets, Earth’s orbital path is an ellipse so the planet is sometimes farther away from the Sun than at other times.
• The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd and the furthest is at aphelion (152 million km) on July 4th.
• Earth’s elliptical orbit has nothing to do with Earth’s seasons.
• During one revolution around the Sun, Earth travels at an average distance of about 150 million km.
• Earth revolves around the Sun at an average speed of about 27 km (17 mi) per second, but the speed is not constant.
• The planet moves slower when it is at aphelion and faster when it is at perihelion.
• The reason the Earth (or any planet) has seasons is that Earth is tilted 23 1/2oon its axis.
• During the Northern Hemisphere summer the North Pole points toward the Sun, and in the Northern Hemisphere winter the North Pole is tilted away from the Sun.

Summer solstice

• On 21st June, the Northern Hemisphere is tilted towards the sun.
• The rays of the sun fall directly on the Tropic of Cancer.
• As a result, these areas receive more heat.
• The areas near the poles receive less heat as the rays of the sun are slanting.
• The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.
• Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator.
• The longest day and the shortest night at these places occur on 21st June.
• At this time in the Southern Hemisphere all these conditions are reversed.
• It is winter season there.
• The nights are longer than the days.
• This position of the earth is called the Summer Solstice

Winter solstice

• On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it.
• As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the Southern Hemisphere gets light.
• Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights.
• The reverse happens in the Northern Hemisphere.
• This position of the earth is called the Winter Solstice
• On 21st March and September 23rd, direct rays of the sun fall on the equator.
• At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.
• On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere.
• The opposite is the case on 21st March, when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Effects of revolution

• Revolution along with the earth’s tilted axis leads to changing seasons across the hemispheres.
• The speed of the Earth’s revolution has influenced the state of the Earth. On account of the speed of pivot, a diffusive power is made which prompts the straightening of the Earth at shafts and protruding at the middle.
• The Earth’s revolution influences the development of water in the seas. The tides are redirected because of the turn.
• The speed of revolution additionally influences the development of the breeze. Because of revolution, winds and the sea flow redirect to one side in the Northern Hemisphere and to one side in the Southern Hemisphere.

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Essay on Earth

500 words essay on earth.

The earth is the planet that we live on and it is the fifth-largest planet. It is positioned in third place from the Sun. This essay on earth will help you learn all about it in detail. Our earth is the only planet that can sustain humans and other living species. The vital substances such as air, water, and land make it possible.

All About Essay on Earth

The rocks make up the earth that has been around for billions of years. Similarly, water also makes up the earth. In fact, water covers 70% of the surface. It includes the oceans that you see, the rivers, the sea and more.

Thus, the remaining 30% is covered with land. The earth moves around the sun in an orbit and takes around 364 days plus 6 hours to complete one round around it. Thus, we refer to it as a year.

Just like revolution, the earth also rotates on its axis within 24 hours that we refer to as a solar day. When rotation is happening, some of the places on the planet face the sun while the others hide from it.

As a result, we get day and night. There are three layers on the earth which we know as the core, mantle and crust. The core is the centre of the earth that is usually very hot. Further, we have the crust that is the outer layer. Finally, between the core and crust, we have the mantle i.e. the middle part.

The layer that we live on is the outer one with the rocks. Earth is home to not just humans but millions of other plants and species. The water and air on the earth make it possible for life to sustain. As the earth is the only livable planet, we must protect it at all costs.

Get the huge list of more than 500 Essay Topics and Ideas

There is No Planet B

The human impact on the planet earth is very dangerous. Through this essay on earth, we wish to make people aware of protecting the earth. There is no balance with nature as human activities are hampering the earth.

Needless to say, we are responsible for the climate crisis that is happening right now. Climate change is getting worse and we need to start getting serious about it. It has a direct impact on our food, air, education, water, and more.

The rising temperature and natural disasters are clear warning signs. Therefore, we need to come together to save the earth and leave a better planet for our future generations.

Being ignorant is not an option anymore. We must spread awareness about the crisis and take preventive measures to protect the earth. We must all plant more trees and avoid using non-biodegradable products.

Further, it is vital to choose sustainable options and use reusable alternatives. We must save the earth to save our future. There is no Planet B and we must start acting like it accordingly.

Conclusion of Essay on Earth

All in all, we must work together to plant more trees and avoid using plastic. It is also important to limit the use of non-renewable resources to give our future generations a better planet.

FAQ on Essay on Earth

Question 1: What is the earth for kids?

Answer 1: Earth is the third farthest planet from the sun. It is bright and bluish in appearance when we see it from outer space. Water covers 70% of the earth while land covers 30%. Moreover, the earth is the only planet that can sustain life.

Question 2: How can we protect the earth?

Answer 2: We can protect the earth by limiting the use of non-renewable resources. Further, we must not waste water and avoid using plastic.

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Difference Between Earth’s Rotation and Revolution

• Categorized under Science | Difference Between Earth’s Rotation and Revolution

Rotation of the earth describes the spinning of the earth around its axis, resulting in the 24 hour phenomenon of day and night over the earth. Revolution on the other hand describes the movement of the earth around the sun over a period of one year, causing seasons to occur. Rotation of the earth causes difference in time over countries and continents. The parts of the earth that are in front of the sun experience day, while the part of the earth away from the sun has night. During the course of revolution depending upon which hemisphere of the earth is closer to the sun, and which one is farther, we have summer and winter respectively. When both the hemispheres are equidistant from the sun there is spring or autumn or fall.

The rotation of the earth around its axis follows the west to east path. The path of the earth around the sun during revolution is an ellipse rather than a circle and this is the reason why the earth is closer to the sun sometimes and farther from it other times, thus causing seasonal fluctuations in weather known as seasons.

The impact of rotation on earth goes far beyond causing day and night. It in fact impacts upon the shape of the earth, which is an oblate spheroid, ocean depth and tectonic plate movement. The earth rotates around its axis at approximately 15 angular degrees per hour. For one complete revolution the earth takes 365.25 days in a slightly elliptical orbit, which has the sun at one focal point of the ellipse.

Rotation and revolution are terms often erroneously used interchangeably because they mean the same thing in a literal and literary sense. However in geography and astronomy the two terms have entirely different meanings and connotations. From a practical point of view the implications of the two phenomena are enormous. Getting a fix on the earth’s various time zones, the study of tides and seismic activity: all of these are interlined with the earth’s rotation. The climatic seasons on the other hand depend entirely on revolution and this helps us anticipate and prepare for the changes.

From a school child’s perspective, it is fascinating to discover that because of rotation one can have night in India and daytime in the US at the same instant. They would also find it incredible that while it is the peak of winter in London in the month of December, there would be summer in Cape Town in the same month. In fact most cultural, demographic and ecological differences that abound upon this beautiful planet of ours are directly linked to these two celestial phenomena. While we go about our daily lives do we ever for an instant imagine that we are hurtling through space perched on a rock that spins around itself, while tracing a path around its sun, which itself is part of vast galaxy of stars, one among millions and millions of other galaxies found in the universe. Truly the mind boggles.

Summary: 1.Rotation of earth takes place on its axis while revolution is its motion around sun. 2.Rotation is completed in 24 hrs while revolution takes 365 days for completion. 3.Due to rotation, there is a difference in the times across nations in the world.

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Cite APA 7 Kumar, M. (2011, October 11). Difference Between Earth’s Rotation and Revolution. Difference Between Similar Terms and Objects. http://www.differencebetween.net/science/difference-between-earth%e2%80%99s-rotation-and-revolution/. MLA 8 Kumar, Manisha. "Difference Between Earth’s Rotation and Revolution." Difference Between Similar Terms and Objects, 11 October, 2011, http://www.differencebetween.net/science/difference-between-earth%e2%80%99s-rotation-and-revolution/.

31 Comments

i still dont understand why the earth is tilted.

why wont the sun must be tilted but not the earth? i know its a simple question, but still dont know it.

As the phase of the sun is in neutral gravity position it doesn’t move while earth is on the magnetic field which tends to move. Thats the answer I think

The reason why the earth is titled is because of the seasons.If the earth did not tilt it would also be the same season

because it is

it means that if the earth was not tilted then there would be no seasons

This article in rotation versus revolution is SO WRONG. The seasons of the Earth are NOT because the orbit of the Earth is elliptical. It is because the Earth is tilted towards or away from the sun during its revolution! The orbit of the Earth around the sun is also almost a perfect circle! Get rid of your terrible facts on this before one of my students read this and get incorrect information.

Please calm down

Well i still dont get this article at ALL!

The earth surely and certainly revolve around sun in an elliptical orbit whose one focus is sun. Kepler had proved it beyond a shadow of doubt. Don’t live in the mediaeval period and let your student learn the right things and avoid a bad pedagogy

how is this wrong? I don’t understand and I’m in 8 grande. pls help me to understand.

brain malfunction too much bull crap

Shut ur mouth, if you dont understand this then there are plently of other sites that you could look on. This doesnt go into major details. It gives you the jist of things. You dont need 2 complain just cause it doesnt have the things yall need.

hey i need to know how does earth have its rotation

It’s kind of like the moon going around the earth because of the gravity pulling it around. Same concept anyway.

I don’seem 2 understand tis article’s

dunt understund grammer

Pls dis site is for searching if u don’t understand the results it doesn’t mean u should say something wrong bcos someone understand. u can visit wikipedia and so on.

I understood even though I am a student of form 3 A & I sometimes have difficulties.

I’m sorry if you are confused, but it is not that hard to comprehend. It just goes into very vivid details. I understand this and I am only in 7th grade!

what would u prefer. big black dick for 1 hour or your least fav teacher for a wife

i would get a gun and kill every bannana in da wurld *DEEP VOICE*MWAHAHAHAHAHAHAHAH

I am stuned i need to now how it works. i have proget and this is one of the things i need to now it count for 80% of my grand and counts 3 times. I need to know if this website is right or wrong i really need 3 of these 100.

how do you not understand this article is easy and very understandable.

I wish to be informed on the day to day happenings on the changes

IT IS MESSED UP!

Describe the diagram of the rotation and revolution

i like it when the earth spins 😛

do you still like it when the earth spins

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Written by : Manisha Kumar. and updated on 2011, October 11 Articles on DifferenceBetween.net are general information, and are not intended to substitute for professional advice. The information is "AS IS", "WITH ALL FAULTS". User assumes all risk of use, damage, or injury. You agree that we have no liability for any damages.

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July 1, 2005

19 min read

Evolution of Earth

The evolution of this planet and its atmosphere gave rise to life, which shaped Earth's subsequent development. Our future lies in interpreting this geologic past and considering what changes--good and bad--may lie ahead

By Claude J. Allègre & Stephen H. Schneider

Like the lapis lazuli gem it resembles, the blue, cloud-enveloped planet the we recognize immediately from satellite pictures seems remarkably stable. Continents and oceans, encircled by an oxygen-rich atmosphere, support familiar life-forms. Yet this constancy is an illusion produced by the human experience of time. Earth and its atmosphere are continuously altered. Plate tectonics shift the continents, raise mountains and move the ocean floor while processes not fully understood alter the climate.

Such constant change has characterized Earth since its beginning some 4.5 billion years ago. From the outset, heat and gravity shaped the evolution of the planet. These forces were gradually joined by the global effects of the emergence of life. Exploring this past offers us the only possibility of understanding the origin of life and, perhaps, its future.

Scientists used to believe the rocky planets, including Earth, Mercury, Venus and Mars, were created by the rapid gravitational collapse of a dust cloud, a deation giving rise to a dense orb. In the 1960s the Apollo space program changed this view. Studies of moon craters revealed that these gouges were caused by the impact of objects that were in great abundance about 4.5 billion years ago. Thereafter, the number of impacts appeared to have quickly decreased. This observation rejuvenated the theory of accretion postulated by Otto Schmidt. The Russian geophysicist had suggested in 1944 that planets grew in size gradually, step by step.

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According to Schmidt, cosmic dust lumped together to form particulates, particulates became gravel, gravel became small balls, then big balls, then tiny planets, or planetesimals, and, nally, dust became the size of the moon. As the planetesimals became larger, their numbers decreased. Consequently, the number of collisions between planetesimals, or meteorites, decreased. Fewer items available for accretion meant that it took a long time to build up a large planet. A calculation made by George W. Wetherill of the Carnegie Institution of Washington suggests that about 100 million years could pass between the formation of an object measuring 10 kilometers in diameter and an object the size of Earth.

The process of accretion had significant thermal consequences for Earth, consequences that forcefully directed its evolution. Large bodies slamming into the planet produced immense heat in its interior, melting the cosmic dust found there. The resulting furnace--situated some 200 to 400 kilometers underground and called a magma ocean--was active for millions of years, giving rise to volcanic eruptions. When Earth was young, heat at the surface caused by volcanism and lava ows from the interior was intensified by the constant bombardment of huge objects, some of them perhaps the size of the moon or even Mars. No life was possible during this period.

Beyond clarifying that Earth had formed through accretion, the Apollo program compelled scientists to try to reconstruct the subsequent temporal and physical development of the early Earth. This undertaking had been considered impossible by founders of geology, including Charles Lyell, to whom the following phrase is attributed: No vestige of a beginning, no prospect for an end. This statement conveys the idea that the young Earth could not be re-created, because its remnants were destroyed by its very activity. But the development of isotope geology in the 1960s had rendered this view obsolete. Their imaginations red by Apollo and the moon ndings, geochemists began to apply this technique to understand the evolution of Earth.

Dating rocks using so-called radioactive clocks allows geologists to work on old terrains that do not contain fossils. The hands of a radioactive clock are isotopes--atoms of the same element that have different atomic weights--and geologic time is measured by the rate of decay of one isotope into another [see "The Earliest History of the Earth," by Derek York; Scientific American , January 1993]. Among the many clocks, those based on the decay of uranium 238 into lead 206 and of uranium 235 into lead 207 are special. Geochronologists can determine the age of samples by analyzing only the daughter product--in this case, lead--of the radioactive parent, uranium.

Panning for zircons ISOTOPE GEOLOGY has permitted geologists to determine that the accretion of Earth culminated in the differentiation of the planet: the creation of the core--the source of Earth's magnetic field--and the beginning of the atmosphere. In 1953 the classic work of Claire C. Patterson of the California Institute of Technology used the uranium-lead clock to establish an age of 4.55 billion years for Earth and many of the meteorites that formed it. In the early 1990s, however, work by one of us (Allègre) on lead isotopes led to a somewhat new interpretation.

As Patterson argued, some meteorites were indeed formed about 4.56 billion years ago, and their debris constituted Earth. But Earth continued to grow through the bombardment of planetesimals until some 120 million to 150 million years later. At that time--4.44 billion to 4.41 billion years ago--Earth began to retain its atmosphere and create its core. This possibility had already been suggested by Bruce R. Doe and Robert E. Zartman of the U.S. Geological Survey in Denver two decades ago and is in agreement with Wetherills estimates.

The emergence of the continents came somewhat later. According to the theory of plate tectonics, these landmasses are the only part of Earth's crust that is not recycled and, consequently, destroyed during the geothermal cycle driven by the convection in the mantle. Continents thus provide a form of memory because the record of early life can be read in their rocks. Geologic activity, however, including plate tectonics, erosion and metamorphism, has destroyed almost all the ancient rocks. Very few fragments have survived this geologic machine.

Nevertheless, in recent decades, several important nds have been made, again using isotope geochemistry. One group, led by Stephen Moorbath of the University of Oxford, discovered terrain in West Greenland that is between 3.7 billion and 3.8 billion years old. In addition, Samuel A. Bowring of the Massachusetts Institute of Technology explored a small area in North America--the Acasta gneiss--that is thought to be 3.96 billion years old.

Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dissolved during the process of erosion but is deposited in particle form in sediment. A few pieces of zircon can therefore survive for billions of years and can serve as a witness to Earths more ancient crust. The search for old zircons started in Paris with the work of Annie Vitrac and Jol R. Lancelot, later at the University of Marseille and now at the University of Nmes, respectively, as well as with the efforts of Moorbath and Allgre. It was a group at the Australian National University in Canberra, directed by William Compston, that was nally successful. The team discovered zircons in western Australia that were between 4.1 billion and 4.3 billion years old.

Zircons have been crucial not only for understanding the age of the continents but for determining when life rst appeared. The earliest fossils of undisputed age were found in Australia and South Africa. These relics of blue-green algae are about 3.5 billion years old. Manfred Schidlowski of the Max Planck Institute for Chemistry in Mainz studied the Isua formation in West Greenland and argued that organic matter existed as long ago as 3.8 billion years. Because most of the record of early life has been destroyed by geologic activity, we cannot say exactly when it rst appeared--perhaps it arose very quickly, maybe even 4.2 billion years ago.

Stories from gases ONE OF THE MOST important aspects of the planet's evolution is the formation of the atmosphere, because it is this assemblage of gases that allowed life to crawl out of the oceans and to be sustained. Researchers have hypothesized since the 1950s that the terrestrial atmosphere was created by gases emerging from the interior of the planet. When a volcano spews gases, it is an example of the continuous outgassing, as it is called, of Earth. But scientists have questioned whether this process occurred suddenly--about 4.4 billion years ago when the core differentiated--or whether it took place gradually over time.

To answer this question, Allègre and his colleagues studied the isotopes of rare gases. These gases--including helium, argon and xenon--have the peculiarity of being chemically inert, that is, they do not react in nature with other elements. Two of them are particularly important for atmospheric studies: argon and xenon. Argon has three isotopes, of which argon 40 is created by the decay of potassium 40. Xenon has nine, of which xenon 129 has two different origins. Xenon 129 arose as the result of nucleosynthesis before Earth and solar system were formed. It was also created from the decay of radioactive iodine 129, which does not exist on Earth anymore. This form of iodine was present very early on but has died out since, and xenon 129 has grown at its expense.

Like most couples, both argon 40 and potassium 40 and xenon 129 and iodine 129 have stories to tell. They are excellent chronometers. Although the atmosphere was formed by the outgassing of the mantle, it does not contain any potassium 40 or iodine 129. All argon 40 and xenon 129, formed in Earth and released, are found in the atmosphere today. Xenon was expelled from the mantle and retained in the atmosphere; therefore, the atmosphere-mantle ratio of this element allows us to evaluate the age of differentiation. Argon and xenon trapped in the mantle evolved by the radioactive decay of potassium 40 and iodine 129. Thus, if the total outgassing of the mantle occurred at the beginning of Earths formation, the atmosphere would not contain any argon 40 but would contain xenon 129.

The major challenge facing an investigator who wants to measure such ratios of decay is to obtain high concentrations of rare gases in mantle rocks because they are extremely limited. Fortunately, a natural phenomenon occurs at mid-ocean ridges during which volcanic lava transfers some silicates from the mantle to the surface. The small amounts of gases trapped in mantle minerals rise with the melt to the surface and are concentrated in small vesicles in the outer glassy margin of lava ows. This process serves to concentrate the amounts of mantle gases by a factor of 10 4 or 10 5 . Collecting these rocks by dredging the seaoor and then crushing them under vacuum in a sensitive mass spectrometer allows geochemists to determine the ratios of the isotopes in the mantle. The results are quite surprising. Calculations of the ratios indicate that between 80 and 85 percent of the atmosphere was outgassed during Earths rst one million years; the rest was released slowly but constantly during the next 4.4 billion years.

The composition of this primitive atmosphere was most certainly dominated by carbon dioxide, with nitrogen as the second most abundant gas. Trace amounts of methane, ammonia, sulfur dioxide and hydrochloric acid were also present, but there was no oxygen. Except for the presence of abundant water, the atmosphere was similar to that of Venus or Mars. The details of the evolution of the original atmosphere are debated, particularly because we do not know how strong the sun was at that time. Some facts, however, are not disputed. It is evident that carbon dioxide played a crucial role. In addition, many scientists believe the evolving atmosphere contained sufficient quantities of gases such as ammonia and methane to give rise to organic matter.

Still, the problem of the sun remains unresolved. One hypothesis holds that during the Archean eon, which lasted from about 4.5 billion to 2.5 billion years ago, the suns power was only 75 percent of what it is today. This possibility raises a dilemma: How could life have survived in the relatively cold climate that should accompany a weaker sun? A solution to the faint early sun paradox, as it is called, was offered by Carl Sagan and George Mullen of Cornell University in 1970. The two scientists suggested that methane and ammonia, which are very effective at trapping infrared radiation, were quite abundant. These gases could have created a super-greenhouse effect. The idea was criticized on the basis that such gases were highly reactive and have short lifetimes in the atmosphere.

What controlled co? IN THE LATE 1970s Veerabhadran Ramanathan, now at the Scripps Institution of Oceanography, and Robert D. Cess and Tobias Owen of Stony Brook University proposed another solution. They postulated that there was no need for methane in the early atmosphere because carbon dioxide was abundant enough to bring about the super-greenhouse effect. Again this argument raised a different question: How much carbon dioxide was there in the early atmosphere? Terrestrial carbon dioxide is now buried in carbonate rocks, such as limestone, although it is not clear when it became trapped there. Today calcium carbonate is created primarily during biological activity; in the Archean eon, carbon may have been primarily removed during inorganic reactions.

The rapid outgassing of the planet liberated voluminous quantities of water from the mantle, creating the oceans and the hydrologic cycle. The acids that were probably present in the atmosphere eroded rocks, forming carbonate-rich rocks. The relative importance of such a mechanism is, however, debated. Heinrich D. Holland of Harvard University believes the amount of carbon dioxide in the atmosphere rapidly decreased during the Archean and stayed at a low level.

Understanding the carbon dioxide content of the early atmosphere is pivotal to understanding climatic control. Two conicting camps have put forth ideas on how this process works. The rst group holds that global temperatures and carbon dioxide were controlled by inorganic geochemical feedbacks; the second asserts that they were controlled by biological removal.

James C. G. Walker, James F. Kasting and Paul B. Hays, then at the University of Michigan at Ann Arbor, proposed the inorganic model in 1981. They postulated that levels of the gas were high at the outset of the Archean and did not fall precipitously. The trio suggested that as the climate warmed, more water evaporated, and the hydrologic cycle became more vigorous, increasing precipitation and runoff. The carbon dioxide in the atmosphere mixed with rainwater to create carbonic acid runoff, exposing minerals at the surface to weathering. Silicate minerals combined with carbon that had been in the atmosphere, sequestering it in sedimentary rocks. Less carbon dioxide in the atmosphere meant, in turn, less of a greenhouse effect. The inorganic negative feedback process offset the increase in solar energy.

This solution contrasts with a second paradigm: biological removal. One theory advanced by James E. Lovelock, an originator of the Gaia hypothesis, assumed that photosynthesizing microorganisms, such as phytoplankton, would be very productive in a high carbon dioxide environment. These creatures slowly removed carbon dioxide from the air and oceans, converting it into calcium carbonate sediments. Critics retorted that phytoplankton had not even evolved for most of the time that Earth has had life. (The Gaia hypothesis holds that life on Earth has the capacity to regulate temperature and the composition of Earth's surface and to keep it comfortable for living organisms.)

In the early 1990s Tyler Volk of New York University and David W. Schwartzman of Howard University proposed another Gaian solution. They noted that bacteria increase carbon dioxide content in soils by breaking down organic matter and by generating humic acids. Both activities accelerate weathering, removing carbon dioxide from the atmosphere. On this point, however, the controversy becomes acute. Some geochemists, including Kasting, now at Pennsylvania State University, and Holland, postulate that while life may account for some carbon dioxide removal after the Archean, inorganic geochemical processes can explain most of the sequestering. These researchers view life as a rather weak climatic stabilizing mechanism for the bulk of geologic time.

Oxygen from algae THE ISSUE OF CARBON remains critical to how life inuenced the atmosphere. Carbon burial is a key to the vital process of building up atmospheric oxygen concentrations--a prerequisite for the development of certain life-forms. In addition, global warming is taking place now as a result of humans releasing this carbon. For one billion or two billion years, algae in the oceans produced oxygen. But because this gas is highly reactive and because there were many reduced minerals in the ancient oceans--iron, for example, is easily oxidized--much of the oxygen produced by living creatures simply got used up before it could reach the atmosphere, where it would have encountered gases that would react with it.

Even if evolutionary processes had given rise to more complicated life-forms during this anaerobic era, they would have had no oxygen. Furthermore, un ltered ultraviolet sunlight would have likely killed them if they left the ocean. Researchers such as Walker and Preston Cloud, then at the University of California at Santa Barbara, have suggested that only about two billion years ago, after most of the reduced minerals in the sea were oxidized, did atmospheric oxygen accumulate. Between one billion and two billion years ago oxygen reached current levels, creating a niche for evolving life.

By examining the stability of certain minerals, such as iron oxide or uranium oxide, Holland has shown that the oxygen content of the Archean atmosphere was low before two billion years ago. It is largely agreed that the present-day oxygen content of 20 percent is the result of photosynthetic activity. Still, the question is whether the oxygen content in the atmosphere increased gradually over time or suddenly. Recent studies indicate that the increase of oxygen started abruptly between 2.1 billion and 2.03 billion years ago and that the present situation was reached 1.5 billion years ago.

The presence of oxygen in the atmosphere had another major bene t for an organism trying to live at or above the surface: it ltered ultraviolet radiation. Ultraviolet radiation breaks down many molecules--from DNA and oxygen to the chlorouorocarbons that are implicated in stratospheric ozone depletion. Such energy splits oxygen into the highly unstable atomic form O, which can combine back into O 2 and into the very special molecule O 3 , or ozone. Ozone, in turn, absorbs ultraviolet radiation. It was not until oxygen was abundant enough in the atmosphere to allow the formation of ozone that life even had a chance to get a root-hold or a foothold on land. It is not a coincidence that the rapid evolution of life from prokaryotes (single-celled organisms with no nucleus) to eukaryotes (single-celled organisms with a nucleus) to metazoa (multicelled organisms) took place in the billion-year-long era of oxygen and ozone.

Although the atmosphere was reaching a fairly stable level of oxygen during this period, the climate was hardly uniform. There were long stages of relative warmth or coolness during the transition to modern geologic time. The composition of fossil plankton shells that lived near the ocean oor provides a measure of bottom water temperatures. The record suggests that over the past 100 million years bottom waters cooled by nearly 15 degrees Celsius. Sea levels dropped by hundreds of meters, and continents drifted apart. Inland seas mostly disappeared, and the climate cooled an average of 10 to 15 degrees C. Roughly 20 million years ago permanent ice appears to have built up on Antarctica.

About two million to three million years ago the paleoclimatic record starts to show signi cant expansions and contractions of warm and cold periods in 40,000-year or so cycles. This periodicity is interesting because it corresponds to the time it takes Earth to complete an oscillation of the tilt of its axis of rotation. It has long been speculated, and recently calculated, that known changes in orbital geometry could alter the amount of sunlight coming in between winter and summer by about 10 percent or so and could be responsible for initiating or ending ice ages.

The warm hand of man MOST INTERESTING and perplexing is the discovery that between 600,000 and 800,000 years ago the dominant cycle switched from 40,000-year periods to 100,000-year intervals with very large uctuations. The last major phase of glaciation ended about 10,000 years ago. At its height 20,000 years ago, ice sheets about two kilometers thick covered much of northern Europe and North America. Glaciers expanded in high plateaus and mountains throughout the world. Enough ice was locked up on land to cause sea levels to drop more than 100 meters below where they are today. Massive ice sheets scoured the land and revamped the ecological face of Earth, which was ve degrees C cooler on average than it is currently.

The precise causes of the longer intervals between warm and cold periods are not yet sorted out. Volcanic eruptions may have played a signi cant role, as shown by the effect of El Chichón in Mexico and Mount Pinatubo in the Philippines. Tectonic events, such as the development of the Himalayas, may have inuenced world climate. Even the impact of comets can inuence short-term climatic trends with catastrophic consequences for life [see "What Caused the Mass Extinction? An Extraterrestrial Impact," by Walter Alvarez and Frank Asaro; and "What Caused the Mass Extinction? A Volcanic Eruption," by Vincent E. Courtillot; Scientific American , October 1990]. It is remarkable that despite violent, episodic perturbations, the climate has been buffered enough to sustain life for 3.5 billion years.

One of the most pivotal climatic discoveries of the past 30 years has come from ice cores in Greenland and Antarctica. When snow falls on these frozen continents, the air between the snow grains is trapped as bubbles. The snow is gradually compressed into ice, along with its captured gases. Some of these records can go back more than 500,000 years; scientists can analyze the chemical content of ice and bubbles from sections of ice that lie as deep as 3,600 meters (2.2 miles) below the surface.

The ice-core borers have determined that the air breathed by ancient Egyptians and Anasazi Indians was very similar to that which we inhale today--except for a host of air pollutants introduced over the past 100 or 200 years. Principal among these added gases, or pollutants, are extra carbon dioxide and methane. Since about 1860--the expansion of the Industrial Revolution--carbon dioxide levels in the atmosphere have increased more than 30 percent as a result of industrialization and deforestation; methane levels have more than doubled because of agriculture, land use and energy production. The ability of increased amounts of these gases to trap heat is what drives concerns about climate change in the 21st century [see "The Changing Climate," by Stephen H. Schneider; Scientific American , September 1989].

The ice cores have shown that sustained natural rates of worldwide temperature change are typically about one degree C per millennium. These shifts are still signi cant enough to have radically altered where species live and to have potentially contributed to the extinction of such charismatic megafauna as mammoths and saber-toothed tigers. But a most extraordinary story from the ice cores is not the relative stability of the climate during the past 10,000 years. It appears that during the height of the last ice age 20,000 years ago there was 50 percent less carbon dioxide and less than half as much methane in the air than there has been during our epoch, the Holocene. This nding suggests a positive feedback between carbon dioxide, methane and climatic change.

The reasoning that supports the idea of this destabilizing feedback system goes as follows. When the world was colder, there was less concentration of greenhouse gases, and so less heat was trapped. As Earth warmed up, carbon dioxide and methane levels increased, accelerating the warming. If life had a hand in this story, it would have been to drive, rather than to oppose, climatic change. It appears increasingly likely that when humans became part of this cycle, they, too, helped to accelerate warming. Such warming has been especially pronounced since the mid-1800s because of greenhouse gas emissions from industrialization, land-use change and other phenomena. Once again, though, uncertainties remain.

Nevertheless, most scientists would agree that life could well be the principal factor in the positive feedback between climatic change and greenhouse gases. There was a rapid rise in average global surface temperature at the end of the 20th century [ see illustration on opposite page ]. Indeed, the period from the 1980s onward has been the warmest of the past 2,000 years. Nineteen of the 20 warmest years on record have occurred since 1980, and the 12 warmest have all occurred since 1990. The all-time record high year was 1998, and 2002 and 2003 were in second and third places, respectively. There is good reason to believe that the decade of the 1990s would have been even hotter had not Mount Pinatubo erupted: this volcano put enough dust into the high atmosphere to block some incident sunlight, causing global cooling of a few tenths of a degree for several years.

Could the warming of the past 140 years have occurred naturally? With ever increasing certainty, the answer is no.

The box at the right shows a remarkable study that attempted to push back the Northern Hemisphere's temperature record a full 1,000 years. Climatologist Michael Mann of the University of Virginia and his colleagues performed a complex statistical analysis involving some 112 different factors related to temperature, including tree rings, the extent of mountain glaciers, changes in coral reefs, sunspot activity and volcanism.

The resulting temperature record is a reconstruction of what might have been obtained had thermometer-based measurements been available. (Actual temperature measurements are used for the years after 1860.) As shown by the confidence range, there is considerable uncertainty in each year of this 1,000-year temperature reconstruction. But the overall trend is clear: a gradual temperature decrease over the first 900 years, followed by a sharp temperature upturn in the 20th century. This graph suggests that the decade of the 1990s was not only the warmest of the century but of the entire past millennium.

By studying the transition from the high carbon dioxide, low-oxygen atmosphere of the Archean to the era of great evolutionary progress about half a billion years ago, it becomes clear that life may have been a factor in the stabilization of climate. In another example--during the ice ages and interglacial cycles--life seems to have the opposite function: accelerating the change rather than diminishing it. This observation has led one of us (Schneider) to contend that climate and life coevolved rather than life serving solely as a negative feedback on climate.

If we humans consider ourselves part of life--that is, part of the natural system--then it could be argued that our collective impact on Earth means we may have a signi cant co-evolutionary role in the future of the planet. The current trends of population growth, the demands for increased standards of living and the use of technology and organizations to attain these growth-oriented goals all contribute to pollution. When the price of polluting is low and the atmosphere is used as a free sewer, carbon dioxide, methane, chlorouorocarbons, nitrous oxides, sulfur oxides and other toxics can build up.

Drastic changes ahead IN THEIR REPORT Climate Change 2001 , climate experts on the Intergovernmental Panel on Climate Change estimated that the world will warm between 1.4 and 5.8 degrees C by 2100. The mild end of that range--a warming rate of 1.4 degrees C per 100 years--is still 14 times faster than the one degree C per 1,000 years that historically has been the average rate of natural change on a global scale. Should the higher end of the range occur, then we could see rates of climatic change nearly 60 times faster than natural average conditions, which could lead to changes that many would consider dangerous. Change at this rate would almost certainly force many species to attempt to move their ranges, just as they did from the ice age/interglacial transition between 10,000 and 15,000 years ago. Not only would species have to respond to climatic change at rates 14 to 60 times faster, but few would have undisturbed, open migration routes as they did at the end of the ice age and the onset of the interglacial era. The negative effects of this significant warming--on health, agriculture, coastal geography and heritage sites, to name a few--could also be severe.

To make the critical projections of future climatic change needed to understand the fate of ecosystems on Earth, we must dig through land, sea and ice to learn as much from geologic, paleoclimatic and paleoecological records as we can. These records provide the backdrop against which to calibrate the crude instruments we must use to peer into a shadowy environmental future, a future increasingly inuenced by us.

THE AUTHORS CLAUDE J. ALLGRE and STEPHEN H. SCHNEIDER study various aspects of Earths geologic history and its climate. Allgre is professor at the University of Paris and directs the department of geochemistry at the Paris Geophysical Institute. He is a foreign member of the National Academy of Sciences. Schneider is professor in the department of biological sciences at Stanford University and co-director of the Center for Environmental Science and Policy. He was honored with a MacArthur Prize Fellowship in 1992 and was elected to membership in the National Academy of Sciences in 2002.

English Compositions

Short Essay on Our Planet Earth [100, 200, 400 words] With PDF

Earth is the only planet that sustains life and ecosystems. In this lesson, you will learn to write essays in three different sets on the planet earth to help you in preparing for your upcoming examinations.

Short Essay on Our Planet Earth in 100 Words

Earth is a rare planet since it is the only one that can support life. On Earth, life is possible for various reasons, the most essential of which are the availability of water and the presence of oxygen. Earth is a member of the Solar System. The Earth, along with the other seven planets, orbits the Sun.

One spin takes approximately twenty-four hours, and one revolution takes 365 days and four hours. Day and night, as well as the changing of seasons, occurs due to rotation and revolution. However, we have jeopardized our planet by our sheer ignorance and negligence. We must practise conservation of resources and look after mother earth while we have time.

Short Essay on Our Planet Earth in 200 Words

Earth is a blue planet that is special from the rest of the planets because it is the only one to sustain life. The availability of water and oxygen are two of the most crucial factors that make life possible on Earth. The Earth rotates around the Sun, along with seven other planets in the solar system. It takes 24 hours to complete one rotation, and approximately 365 days and 4 hours to complete one revolution. Day and night, as well as changing seasons, are all conceivable due to these two movements. 

However, we are wasting and taking advantage of the natural resources that have been bestowed upon us. Overuse and exploitation of all-natural resources produce pollution to such an alarming degree that life on Earth is on the verge of extinction. The depletion of the ozone layer has resulted in global warming. The melting of glaciers has resulted in rising temperatures.

Many animals have become extinct or are endangered. To protect the environment, we must work together. Conversation, resource reduction, reuse, and recycling will take us a long way toward restoring the natural ecosystem. We are as unique as our home planet. We have superior intelligence, which we must employ for the benefit of all living beings. The Earth is our natural home, and we must create a place that is as good as, if not better than, paradise.

Short Essay on Our Planet Earth in 400 Words

Earth is a unique planet as it is the only planet that sustains life. Life is possible on Earth because of many reasons, and the most important among them is the availability of water and oxygen. Earth is a part of the family of the Sun. It belongs to the Solar System.

Earth, along with seven other planets, revolves around the Sun. It takes roughly twenty-four hours to complete one rotation and 365 days and 4 hours to complete one revolution. Rotation and revolution make day and night and change of seasons simultaneously possible. The five seasons we experience in one revolution are Spring, Summer, Monsoon, Autumn, and Winter.

However, we are misusing resources and exploiting the natural gifts that have been so heavily endowed upon us. Overuse and misuse of all the natural resources are causing pollution to such an extent that it has become alarming to the point of destruction. The most common form of pollution caused upon the earth by us is Air Pollution, Land Pollution, Water Pollution, and Noise Pollution.

This, in turn, had resulted in Ozone Layer Depletion and Global Warming. Due to ozone layer depletion, there harmful ultraviolet rays of the sun are reaching the earth. It, in turn, is melting glaciers and causing a rise in temperature every year. Many animals have either extinct or are endangered due to human activities.

Some extinct animals worldwide are Sabre-toothed Cat, Woolly Mammoth, Dodo, Great Auk, Stellers Sea Cow, Tasmanian Tiger, Passenger Pigeon, Pyrenean Ibex. The extinct animals in the Indian subcontinent are the Indian Cheetah, pink-headed duck, northern Sumatran rhinoceros, and Sunderban dwarf rhinoceros.

The endangered animals that are in need of our immediate attention in India are Royal Bengal Tiger, Snow leopard, Red panda, Indian rhinoceros, Nilgiri tahr, Asiatic lion, Ganges river dolphin, Gharial and Hangul, among others. We have exploited fossil fuels to such an extent that now we run the risk of using them completely. We must switch to alternative sources of energy that are nature friendly. Solar power, windmills, hydra power should be used more often, and deforestation must be made illegal worldwide.

We must come together to preserve the natural environment. Conversation, reduction, reuse and recycling of the resources will take us a long way in rebuilding the natural habitat. We are as unique as our planet earth. We have higher intelligence, and we must use it for the well-being of all living organisms. The Earth is our natural abode, and we must make a place as close to Paradise, if not better.

Hopefully, after going through this lesson, you have a holistic idea about our planet Earth. I have tried to cover every aspect that makes it unique and the reasons to practise conversation of natural resources. If you still have any doubts regarding this session, kindly let me know through the comment section below. To read more such essays on many important topics, keep browsing our website. 

Join us on Telegram to get the latest updates on our upcoming sessions. Thank you, see you again soon.

Earth's Changing Climate

Climate change is a long-term shift in global or regional climate patterns. Often climate change refers specifically to the rise in global temperatures from the mid 20th century to present.

Earth Science, Geography, Human Geography, Physical Geography

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Climate  is the long-term pattern of weather in a particular area. Weather can change from hour to hour, day to day, month to month or even from year to year. For periods of 30 years or more, however, distinct  weather patterns occur. A  desert  might experience a rainy week, but over the long term, the region receives very little rainfall . It has a dry climate .

Because climates are mostly constant, living things can  adapt  to them. Polar bears have adapted to stay warm in  polar climates , while cacti have evolved to hold onto water in  dry climates . The  enormous  variety of life on Earth results in large part from the variety of climates that exist.

Climates do change, however—they just change very slowly, over hundreds or even thousands of years. As climates change, organisms that live in the area must adapt ,  relocate , or risk going  extinct .

Earth’s Changing Climate Earth’s climate has changed many times. For example,  fossils from the Cretaceous period (144 to 65 million years ago) show that Earth was much warmer than it is today.  Fossilized plants and animals that normally live in warm  environments have been found at much higher  latitudes than they could survive at today. For instance,  breadfruit  trees ( Artocarpus altilis ), now found on  tropical   islands , grew as far north as Greenland.

Earth has also experienced several major  ice ages —at least four in the past 500,000 years. During these periods, Earth’s  temperature   decreased , causing an  expansion  of  ice sheets and  glaciers . The most recent Ice Age began about two million years ago and peaked about 20,000 years ago. The ice caps began retreating 18,000 years ago. They have not disappeared completely, however. Their presence in Antarctica and Greenland suggests Earth is still in a sort of ice age. Many scientists believe we are in an  interglacial period , when warmer temperatures have caused the ice caps to  recede . Many centuries from now, the glaciers may advance again. Climatologists look for evidence of past climate change in many different places. Like clumsy criminals, glaciers leave many clues behind them. They scratch and  scour   rocks as they move. They deposit sediment  known as glacial till. This sediment sometimes forms mounds or ridges called  moraines . Glaciers also form elongated oval hills known as  drumlins . All of these geographic features on land that currently has no glaciers suggest that glaciers were once there. Scientists also have chemical evidence of ice ages from sediments and  sedimentary rocks . Some rocks only form from glacial material. Their presence under the ocean or on land also tells scientists that glaciers were once present in these areas. Scientists also have paleontological evidence—fossils. Fossils show what kinds of animals and plants lived in certain areas. During ice ages, organisms that are adapted to cold weather can increase their range , moving closer to the  Equator . Organisms that are adapted to warm weather may lose part of their  habitat , or even go extinct.

Climate changes occur over shorter periods, as well. For example, the so-called  Little Ice Age  lasted only a few hundred years, peaking during the 16th and 17th centuries. During this time, average global temperatures were 1 to 1.5 degrees Celsius (2 to 3 degrees Fahrenheit) cooler than they are today.

A change of one or two degrees might not seem like a lot, but it was enough to cause some pretty massive effects. For instance, glaciers grew larger and sometimes engulfed whole mountain villages. Winters were longer than usual, limiting the growing seasons of  crops . In northern Europe, people deserted farms and villages to avoid  starvation .

One way scientists have learned about the Little Ice Age is by studying the rings of trees that are more than 300 years old. The thickness of  tree rings is related to the amount of the trees’ annual growth. This in turn is related to climate changes. During times of  drought  or cold, trees could not grow as much. The rings would be closer together.

Some climate changes are almost predictable . One example of regular climate change results from the warming of the surface waters of the tropical eastern Pacific Ocean. This warming is called El Niño —The Child—because it tends to begin around Christmas. In normal years,  trade winds blow steadily across the ocean from east to west, dragging warm surface water along in the same direction. This produces a shallow layer of warm water in the eastern Pacific and a buildup of warm water in the west. Every few years, normal winds falter and ocean currents reverse. This is El Niño. Warm water deepens in the eastern Pacific. This, in turn, produces  dramatic  climate changes. Rain decreases in Australia and southern Asia, and freak storms may pound Pacific islands and the west coast of the Americas. Within a year or two, El Niño ends, and climate systems return to normal.

Natural Causes of Climate Change Climate changes happen for a variety of reasons. Some of these reasons have to do with Earth’s  atmosphere . The climate change brought by El Niño, which relies on winds and ocean currents, is an example of natural atmospheric changes. Natural climate change can also be affected by forces outside Earth’s atmosphere. For instance, the 100,000-year cycles of ice ages are probably related to changes in the tilt of Earth’s  axis  and the shape of its  orbit  around the sun. Those planetary factors change slowly over time and affect how much of the sun’s energy reaches different parts of the world in different seasons.

The impact of large  meteorites on Earth could also cause climate change . The impact of a meteor would send millions of tons of  debris  into the atmosphere . This debris would block at least some of the sun’s rays, making it cold and dark. This climate change would severely limit what organisms could survive. Many  paleontologists believe the impact of a meteor or comet contributed to the extinction of the dinosaurs .  Dinosaurs simply could not survive in a cool, dark climate . Their bodies could not adjust to the cold, and the dark limited the growth of plants on which they fed.

Plate tectonics  also play a role in climate changes. Earth’s continental plates have moved a great deal over time. More than 200 million years ago, the continents were  merged together as one giant landmass called  Pangaea . As the continents broke apart and moved, their positions on Earth changed, and so did the movements of ocean currents. Both of these changes had effects on climate. Changes in  greenhouse gases in the atmosphere also have an impact on climate change. Gases like  carbon dioxide  trap the sun’s heat in Earth’s atmosphere, causing temperatures on the surface to rise.  Volcanoes —both on land and under the ocean—release greenhouse gases, so if the eruption only reaches the troposphere the additional gases contribute to warming. However, if the eruption is powerful enough to reach the stratosphere particles reflect sunlight back into space causing periods of cooling regionally.

Human Causes of Climate Change Some human activities release greenhouse gases—burning  fossil fuels for  transportation  and  electricity , or using  technology  that increases meat production, for instance. Trees absorb carbon dioxide, so cutting down forests for  timber  or  development  contributes to the greenhouse effect . So do factories that  emit   pollutants into the atmosphere.

Many scientists are worried that these activities are causing dramatic and dangerous changes in Earth’s climate. Average temperatures around the world have risen since about 1880, when scientists began tracking them. The seven warmest years of the 20th century occurred in the 1990s. This warming trend may be a sign that the greenhouse effect is increasing because of human activity. This climate change is often referred to as “ global warming .” Global warming is often linked to the burning of fossil fuels— coal ,  oil , and  natural gas —by industries and cars. Warming is also linked to the destruction of tropical forests. The University of California Riverside and NASA estimate human activity has increased the amount of carbon dioxide in the atmosphere by about 30 percent in the past 150 years. The amount of methane , a potent greehouse gas produced by decomposing plant and animal matter, is also increasing. Increased amounts of methane in Earth’s atmosphere are usually linked to  agricultural development  and industrial technology. As economies grow, populations consume more goods and throw away more materials. Large landfills , filled with decomposing waste, release tons of methane into the atmosphere. Chlorofluorocarbon (CFC) ,  hydrochlorofluorocarbon  (HCFC), and  hydrofluorocarbon  (HFC) chemicals are used in refrigeration and aerosol sprays. These chemicals are also greenhouse gases. Many countries are working to  phase out  their use, and some have laws to prevent companies from manufacturing them.

Global Warming

As the proportion of greenhouse gases in the atmosphere rises, so does the temperature of Earth. Climatologists worry that the global temperature will increase so much that ice caps will begin melting within the next several decades . This would cause the  sea level  to rise. Coastal areas, including many low-lying islands, would be flooded. Severe climate change may bring more severe weather patterns—more  hurricanes ,  typhoons , and  tornadoes . More precipitation would fall in some places and far less in others. Regions where crops now grow could become deserts. As climates change, so do the habitats for living things. Animals that live in an area may become threatened. Many human societies depend on specific crops for food , clothing, and trade . If the climate of an area changes, the people who live there may no longer be able to grow the crops they depend on for survival. Some scientists worry that as Earth warms,  tropical diseases such as  malaria ,  West Nile virus , and  yellow fever  will expand into more  temperate  regions. The temperature will continue to rise unless preventive steps are taken. Most climatologists agree that we must reduce the amount of greenhouse gases released into the atmosphere. There are many ways to do this, including:

• Drive less. Use  public transportation ,  carpool , walk, or ride a bike.
• Fly less. Airplanes produce huge amounts of greenhouse gas emissions .
• Reduce, reuse, and recycle.
• Plant a tree. Trees absorb carbon dioxide, keeping it out of the atmosphere.
• Use less electricity.
• Eat less meat. Cows are one of the biggest methane producers.
• Support alternative energy sources that don’t burn fossil fuels, such as  solar power  and  wind energy .

The climate has changed many times during Earth’s history, but the changes have occurred slowly, over thousands of years. Only since the Industrial Revolution have human activities begun to influence climate—and scientists are still working to understand what the  consequences might be.

Cool Warming Could the current phase of climate change cause another Little Ice Age? As strange as it sounds, some scientists believe it could. If melting glaciers release large amounts of freshwater into the oceans, this could disrupt the ocean conveyor belt, an important circulation system that moves seawater around the globe. Stopping this cycle could possibly cause cooling of 3 to 5 degrees Celsius (5-9 degrees Fahrenheit) in the ocean and atmosphere.

Early Squirrels The North American red squirrel has started breeding earlier in the year as a result of climate change. Food becomes available to the squirrels earlier because of warmer winters.

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Ruins at Tikal National Park, Guatemala. Photo by Mario Bollini/Flickr

Beyond kingdoms and empires

A revolution in archaeology is transforming our picture of past populations and the scope of human freedoms.

by David Wengrow   + BIO

Contemporary historians tell us that, by the start of the Common Era, approximately three-quarters of the world’s population were living in just four empires (we’ve all heard of the Romans and the Han; fewer of us, perhaps, of the Parthians and Kushans). Just think about this for a minute. If true, then it means that the great majority of people who ever existed were born, lived and died under imperial rule. Such claims are hardly original, but for those who share Arnold Toynbee’s conviction that history should amount to more than ‘just one damned thing after another’, they have taken on a new importance.

For some scholars today, the claims prove that empires are obvious and natural structures for human beings to inhabit, or even attractive political projects that, once discovered, we have reproduced again and again over the longue durée of history. The suggestion is that if the subjects of empire in times past could have escaped, they’d have been unwise to do so, and anyway the majority would have preferred life in imperial cages to whatever lay beyond, in the forest or marshes, in the mountains and foothills, or out on the open steppe. Such ideas have deep roots, which may be one reason why they often go unchallenged.

The temple complex at Tikal National Park, Guatemala. Photo by Ryanacandee/Flickr

In the late 18th century, Edward Gibbon – taking inspiration from ancient writers such as Tacitus – described the Roman Empire (before its ‘decline and fall’) as covering ‘the fairest part of the earth, and the most civilised portion of mankind,’ surrounded by barbarians whose freedom was little more than a side-effect of their primitive ways of life. Gibbon’s barbarian is an inveterate idler: free, yes, but only to live in scattered homesteads, wearing skins for clothes, or following his ‘monstrous herds of cattle’. ‘Their poverty,’ wrote Gibbon of the ancient Germans, ‘secured their freedom.’

It is from such sources that we get, not just our notion of empire as handmaiden to civilisation, but also our contemporary image of life before and beyond empire as being small-scale, chaotic and largely unproductive. In short, everything that is still implied by the word ‘tribal’. Tribes are to empires (and their scholarly champions) much as children were to adults of Gibbon’s generation – occasionally charming or amusing creatures, but mostly a disruptive force, whose destiny is to be disciplined, put to useful work, and governed, at least until they are ready to govern themselves in a similar fashion. Either that, or to be confined, punished and, if necessary, eliminated from the pages of history.

Ideas of this sort are, in fact, as old as empire itself. In their diplomatic correspondence, which can be followed back to a time more than 3,000 years ago, the rulers of Egypt and Syria grumble incessantly about the subversive activities of groups called ʿ Apiru . Scholars of the ancient Near East once took ʿ Apiru to be an early reference to the Hebrews, but it’s now thought to be an umbrella term, used almost indiscriminately for any group of political defectors, dissenters, insurgents or refugees who threatened the interests of Egypt’s vassals in neighbouring Canaan (much as some modern politicians have been known to use the word ‘terrorist’ for rhetorical effect today).

In Babylonia, such groups – when not given tribal or ethnic labels – might be variously described as ‘scattered people’, ‘head-bangers’ or simply ‘enemies’. In the early centuries BCE, emissaries of the Han Empire wrote in similar ways about the rebellious marsh-dwellers of the tropical coastlands to their south. Historians now see these ancient inhabitants of Guangdong and Fujian through Han eyes, as the ‘Bai-yue’ (‘Hundred Yue’), who were said to shave their heads, cover their bodies in tattoos, and sacrifice live humans to their savage gods. After centuries of resistance and guerrilla warfare, we learn, the Yue capitulated. On the order of Emperor Wu, most were deported and put to hard labour, their lands given over to colonial settlers from the north, including many retired soldiers.

E mpires have always created vivid and disturbingly violent images of tribal life on their frontiers, placing in a different, paternalistic light the violence at the heart of their own political projects. In such ways, we convince ourselves that these things are somehow deeply related, that violence and domination are the necessary substratum of ‘civilisation’, or that Europe after the fall of Rome achieved something unique – unnatural even, on a global scale – by breaking decisively from ancient cycles of empire and forging a singular path to liberty and prosperity. Once entrenched in our imaginations, such ways of thinking are fiendishly difficult to reverse. Even experienced scholars of empirically grounded disciplines may find themselves advancing such arguments based on the flimsiest of sources.

According to Walter Scheidel, a professor of classics and history at Stanford University in California, the population figures cited at the start of this essay ‘convey a sense of the competitive advantage of a particular type of state: far-flung imperial structures held together by powerful extractive elites.’ In ‘quantitative terms,’ he tells us in The Great Leveller (2017), this ‘proved extremely successful.’ Looking deeper back in time, to the very ‘origin of the state’, Scheidel further conjectures that ‘3,500 years ago, when state-level polities covered perhaps not more than 1 per cent of the earth’s terrestrial surface (excluding Antarctica), they already laid claim to up to half of our species.’

Venture down into the footnotes, and you discover that everyone is citing the same source

Now, it is surely true that in any period of human history, there will always be those who feel most comfortable in ranks and orders. As Étienne de La Boétie had already pointed out in the 16th century, the source of ‘voluntary servitude’ is arguably the most important political question of them all. But where do the statistics come from, to support such grand claims? Are they reliable? Venture down into the footnotes, and you discover that everyone is citing the same source: an Atlas of World Population History , published in 1978; in fairness, Scheidel does provide one additional citation, to Joel Cohen’s How Many People Can the Earth Support? (1995), but this turns out to comprise a chart showing estimates of past human population sizes in which all figures for the premodern era derive from, again, the Atlas of World Population History or from subsequent publications based on it.

In light of all this, anyone today who consults the Atlas of World Population History for the first time is in for a surprise. It is an unassuming tome, and a very old one at that. It comprises simple-to-read population graphs for different world regions, accompanied by pithy essays, which sometimes verge on the laconic. There is also an Appendix on ‘Reliability’ that begins: ‘The hypotheses of the historical demographer are not, in the current state of the art, testable and consequently the idea of their being reliable in the statistician’s sense is out of the question.’

I’ll come back to this important point in a moment.

F irst, it is worth reflecting on what it means to talk of the ‘competitive advantage’ of states governed by extractive elites. At the least, this introduces questions about the long-term strength and viability of certain types of society, and the endemic weaknesses of others. Only the ‘winners’, one assumes, get to forge viable paths to the future. These, however, are matters of opinion, not statistics. They brush over irritating questions like ‘How many benefited from living in imperial structures?’ or ‘What is a viable path into the future?’ What ‘advantages’, we might ask, accrued to a girl captured by Cilician pirates, and sold in the slave markets of Roman-era Delos, over one living freely in the Nuba Hills of southern Kordofan? Slaves were said to change hands at a rate of 10,000 per day in Delos, and the total number of slaves in the early Roman Empire could have been between 6 and 10 million. In what sense can their numbers be added to the side of the ‘winners’?

Thinking ‘in quantitative terms’ doesn’t really allow us to bypass these issues, or at least it shouldn’t. Questions remain. What, exactly, were ancient empires ‘successful’ at, if extraordinary levels of violence, destruction and displacement were required to keep them afloat? Today it seems very possible that another 2,000 years of world governance by ‘powerful extractive elites’ could lead to the destruction of most life on Earth. Many experts think it could happen far sooner if we simply continue with the status quo. Looking back from such a vantage point, if anyone will even be able to do that, who then will seem to be the ‘winners’ of history? Will history have made losers of us all? Would the ‘fittest’ have found an exit route, some way to terraform other parts of the solar system, founding colonies on Mars or Venus that resemble Palo Alto, or even Massachusetts? If there are schools (or at least, TED talks), then perhaps future generations will look back and ask if we might have learned something from the faded traditions of those who lived otherwise. What if there is no Planet B ? Or maybe, by then, none of it will really matter very much, because the past will itself have been automated. Instead of historians, we’ll have ‘history machines’ based on algorithms and databanks: more facts on file, designed by survivors of the final bureaucratic assault on what was once fondly called ‘the humanities’.

Let’s come back to the figures in the Atlas of World Population History. It estimates 46 million Roman subjects and 50 million Han subjects. Let’s assume, for a moment, this is OK. Supposedly – if we combine it with statistics for other empires of the same era – this amounts to ‘between two-thirds and three-quarters of all people alive at the time’ (to quote Scheidel). But what does the Atlas tell us about all those other parts of the ancient world that lay beyond the grasp of powerful, extractive elites? Were they really so empty?

New techniques are revealing entire traditions of urban life, spanning centuries or even millennia

One way to control the quality of historical conjecture is by using rigorous sources that are up to date. In the social sciences, basing important claims on a source from 1978 is going back a while (I was six years old when the Atlas in question was published). Surely such significant matters have been the subject of continuing research? Over the past 45 years, scholars’ understanding of imperial Roman and Chinese demography may not have changed that much. It has, after all, been a matter of serious research for decades. But we can hardly claim that there’s been no relevant, even vital progress in our knowledge of other parts of the world. This is especially true in my own field of archaeology.

Over the past few decades, geographical spaces once written off as blanks on the map, or dismissed as ‘an unchanging palaeolithic backwater’ (as our 1978 Atlas puts it, for Aboriginal Australia), have been flooded with new data. Archaeology, specifically rapid advances in settlement archaeology and methods of survey, has been one major contributor. Among other things, these new techniques are revealing entire traditions of urban life, spanning centuries or even millennia, where none were previously suspected. All of them lie within the scope of the past 5,000 years, but surprisingly few can be convincingly identified with the rise of bureaucratically ordered kingdoms or empires.

In the years following the publication of the Atlas , archaeologists working in the inland delta of the Middle Niger revealed evidence for a prosperous urban civilisation with no discernible signs of rulership or central authority, focused on the site of Jenne-jeno, and preceding the empires of Ghana, Mali and Songhai by some centuries. China, too, has gained a long history of cities before empire, from the lower reaches of the Yellow River to the Fen Valley of Shanxi province, and the ‘Liangzhu culture’ of Jiangsu and Zhejiang. The same is true for the coastlands of Peru, where archaeologists have uncovered huge settlements with sunken plazas and grand platforms, four millennia older than the Inca Empire. In Ukraine, before the Russian invasion, archaeological work on the grasslands north of the Black Sea – which ancient Greek authors portrayed as ‘barbarian steppe’, a land of fierce nomads – was generating detailed evidence of a lost urban tradition, 3,000 years before Herodotus; at sites such as Nebelivka, for example.

As David Graeber and I observed in The Dawn of Everything: A New History of Humanity (2021), our knowledge of such regions and their deep histories has increased exponentially in recent decades. Why, then, beat such a hasty retreat to the state of knowledge as it existed in 1978, and miss out on all this new information? In a moment, we’ll get a clearer sense of what it means to do this, and just how shaky things can get. First, though, there’s more to say on the question of reliability. Colin McEvedy and Richard Jones – who authored the Atlas of World Population History – were, in fact, disarmingly frank about the quality of their data: ‘we wouldn’t attempt to disguise the hypothetical nature of our treatment of the earlier periods … we haven’t just pulled the figures out of the sky. Well, not often.’ Their own acknowledgement of the provisional, hypothetical nature of some of these figures is interesting, since Scheidel is hardly alone in basing some extremely broad assertions on this single, dated source.

L ast year, Timothy Guinnane, professor emeritus of economics at Yale University in Connecticut, decided to blow the whistle on the Atlas .

Writing in The Journal of Economic History , Guinnane assessed the scholarly merits of the Atlas . He deemed it ‘unreliable’ – little more than an encyclopaedia of zombie statistics. Guinnane finds the Atlas guilty of sometimes reporting population estimates with no apparent supporting evidence, or no indication of how a particular source was converted into usable statistics. Historians, he suggests, should stop using it, because claims about global history based on the Atla s can only ever resemble the proverbial piece of old Swiss cheese: mostly missing, and what’s there is off. Its authors, he notes, were often more candid about the gaps in their work, and the speculative nature of their population graphs, than the researchers who continue to use their data.

How did such a dismal source as the Atlas become the sole basis for a demographics of doom?

In a recent interview with the Long Now Foundation, Guinnane explains how he was inspired to write his article after seeing one scholar after another make ‘references to data of this type’. Confronted, time and again, with statistics that have a major bearing on our understanding of world history, including the long-term history of human land use and environmental change, he quite reasonably wanted to know where the population figures come from; much as we might ask how anyone can surmise that, at the start of the Common Era, most people on Earth were living under imperial rule. Why do we think that? In Guinnane’s own words, the results of his enquiries left him ‘baffled’, since his experience indicates that ‘no such data exist’. Hence the title of his journal paper: ‘We Do Not Know the Population of Every Country in the World for the Past Two Thousand Years’ (let alone the past 5,000).

In the near future, intellectual historians may puzzle over how, a half-century after its publication, such a dismal source as the Atlas became the sole basis for a demographics of doom. How, on such feeble foundations, did we pump into our heads the notion that there is something natural about living in societies where rulers exercise arbitrary authority over subjects? Or where dissent is likely to be met with brutality, imprisonment, exile or even death; and where the only alternative is something like the ‘liberty’ of Gibbon’s ancient Germans: a life of freedom, but also of poverty, ignorance and general immiseration.

Guinnane names prominent economic historians who continue to use the Atlas in various ways. As he rightly emphasises, the blame shouldn’t really lie with individual scholars. He is also, surely, correct to say that, so long as reputable publishers of academic books and journals continue to accept work based on such sources, we’re unlikely to see much progress: ‘estimates will not improve unless we care enough about good data to stop naively using the old.’ Archaeology, I suggest, has a vital role to play here.

L et’s take the example of the Amazon rainforest, an area of well over 2 million square miles, with no history of empire until the European conquest, and which the Atlas characterises as yet another demographic backwater, thinly scattered with nomadic foragers, whose mode of livelihood (its authors assumed) could never support dense populations. How does this hold up today?

Lidar map of the city of Kunguints in the Ecuadorian Amazon showing ancient streets lined with houses. Courtesy Antoine Dorison and Stéphen Rostain/Wikipedia

It doesn’t. Over the past decade, archaeologists have been busily turning the whole picture on its head, using airborne lasers to peer through the forest canopy. Tropical landscapes that resisted terrestrial survey are giving up their secrets. In place of blanks on the map, we’re now able to see highly cultivated landscapes with massive infrastructure stretching back to the early centuries BCE. Road networks, terraces, ceremonial earthworks, planned residential neighbourhoods, and regional settlement systems ordered into patterns of geometrical precision can be traced across Amazonia, from Brazil to Bolivia , as far as the eastern foothills of the Andes . In certain parts of Amazonia, the forest itself turns out to be a product of past human interaction with the soil. Over time, this generated the rich ‘anthropogenic’ earths called terra preta de índio (‘black earth of the Indians’), with levels of fertility far in excess of ordinary tropical soils. Scientists now believe that between 10,000 and 20,000 large-scale sites remain to be discovered across Amazonia. Similarly startling finds are emerging from Southeast Asia, and we might reasonably expect them from the forested parts of the African continent too.

Of course, the same procedures are changing our picture of tropical landscapes that did witness the rise and fall of great kingdoms, and even empires. Archaeologists now believe that in the year 500 CE, between 10 and 15 million people lived in the Maya lowlands of Yucatán and northern Guatemala. For comparison, the Atlas offers a figure of just 2 million for all of Mexico in the same era, including the Indigenous cities of the Altiplano (at least some of which, we now know , were organised not as empires or even kingdoms, but fiercely autonomous republics, long before the Spanish conquest).

Until surprisingly recent times, spaces of human freedom existed across large parts of our planet

It is easy, encouraged by works such as the Atlas , to imagine ancient history as a chequerboard of kingdoms and empires. But it is also very misleading. Ancient polities in the Maya lowlands and Southeast Asia had porous boundaries, constantly shifting, and open to contestation. Authority waned with distance from the centre. Warfare and tribute were largely seasonal affairs, after which coercive power shrank back behind the walls of the capital. As the archaeologist Monica Smith points out , only the most naive historian would assume that the claims inscribed on imperial monuments are a simple reflection of political reality on the ground. Of course ancient rulers loved to present themselves as ‘sovereigns of the four quarters’, ‘masters of the known world’, and so on. Yet no ancient world emperor could even have imagined powers of surveillance, such as those now enjoyed by any minor dictator or oligarch.

On a global scale, we are witnessing a revolution in our understanding of ancient demography. To ignore it, these days, is to indulge in a cruel sort of intellectual prank, by which the genocide of Indigenous populations – a direct consequence of the planetary revolt against freedom, in the past 500 years – is naturalised as a perennial absence of people. Nor can we just assume that if we want to understand the prospects for our modern world, the only ‘big’ stories worth telling are those of empire.

The world we live in today is not just the one created by the likes of Tiberius of Rome, or even Emperor Wu of Han. Until surprisingly recent times, spaces of human freedom existed across large parts of our planet. Millions lived in them. We don’t know their names, as they didn’t carve them in stone, but we know that many lived lives in which one could hope to do more than just scratch out an existence, or rehearse someone else’s script of ‘the origin of the state’ – in which one could move away, disobey, experiment with other notions of how to live, even create new forms of social reality.

Sometimes, the unfree did this too, against much harder odds. How many, back then, preferred imperial control to non-imperial freedoms? How many were given a choice? How much choice do we have now? It seems nobody really knows the answers to these questions, at least not yet. In future, it will take more than zombie statistics to stop us from asking them. There are forgotten histories buried in the ground, of human politics and values. The soil mantle of Earth, including the very soil itself, turns out to be not just our species’ life support system, but also a forensic archive, containing precious evidence to challenge timeworn narratives about the origins of inequality, private property, patriarchy, warfare, urban life and the state – narratives born directly from the experience of empire, written by the ‘winners’ of a future that may yet make losers of us all.

Investigating the human past in this way is not a matter of searching for utopia, but of freeing us to think about the true possibilities of human existence. Unhampered by outdated theoretical assumptions and dogmatic interpretations of obsolete data, could we look with fresh eyes at the very meaning of terms like ‘ civilisation ’? Our species has existed for something like 300,000 years. Today, we stand on a precipice, confronting a future defined by environmental collapse, the erosion of democracy, and wars of unprecedented destructiveness: a new age of empire, perhaps the last in a cycle of such ages that, for all we really know, may represent only a modest fraction of the human experience.

For those who seek to change course, such uncertainty about the scope of human freedoms may itself be a source of liberation, opening pathways to other futures.

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