ICSE Class 6 Geography Chapter 1 Earth as a Planet

Chapter One: Earth As A Planet

Syllabus

Earth as a Planet: Shape of the earth. Earth as the home of humankind and the conditions that exist.

The Shape Of The Earth

In the earlier days, it was believed that the earth was the centre of the universe and all the heavenly bodies of the universe were moving around it. However, today it is an accepted fact that the earth is one of the eight planets that move around the sun.

It is the only planet which supports life. From space, the earth looks blue because 70 per cent of its total area is covered by water. Hence, it is also called the watery planet. The family of the sun is called the Solar System. The members of the solar system are at a certain distance from the sun. The chief members of the solar system are the planets, the satellites that revolve around the planets and other celestial bodies called asteroids, comets and meteorites. The planets in order of their distance from the sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The earth is not a perfect sphere. It bulges slightly at the centre and is flattened at the poles. In the older days, the earth was considered to be a flat disc with steep edges. Aryabhatta, the 5th century Indian astronomer and mathematician, believed in the round shape of the earth. He said that the earth was spherical in shape and rotated on its axis. In the 6th century, Greek philosopher Pythagoras said the same thing. Today, we have convincing evidence to prove that the earth is spherical in shape.

Proof That The Earth Is Spherical In Shape

(I) The Bedford Level Experiment: An experiment was carried out in the Bedford Level canal area in England. Three poles of equal length were fixed at intervals of 5 km. It was ensured that the poles had the same height above the water in the canal. When the poles were viewed from one end of the canal, the middle pole looked slightly higher. It could not have happened if the earth had a flat surface (Fig. 1.1). A similar experiment was repeated at different places with the same result. This experiment demonstrated that the surface of the earth was curved.

(II) Sighting of a ship: While standing at seashore, watching an approaching ship, we first see only its mast. Then, as it comes closer, we can see the deck in the middle, then its funnel, and finally the hull comes into full view. This can happen only when the earth's surface is curved (Fig. 1.2). If the earth were flat, the entire ship would have been visible at once.

(III) Pole Star: The Pole star can be seen at an angle of 90° at the North Pole. It lies in line with the axis of the earth, and its angle decreases towards the Equator. At the Equator, the angle is 0°. This can happen only in an arc of a circle. (Fig. 1.3). If the earth were flat, the Pole Star would have the same height for all latitudes.

(IV) Satellite Pictures: Jean Baptiste Delambre (1749-1822), the French astronomer, discovered the flattening of the earth at the poles. In the 20th century, when satellite pictures of earth were fed into computers, its spherical shape with the flattened poles was confirmed (Fig. 1.4)

(V) Lunar Eclipse: The shadow of earth on the surface of the moon is clearly visible from earth during lunar eclipse. It appears as an arc of a circle. Since the earth is bigger than the moon, it is rarely oriented in the same position as the moon but the shadow is always circular.

(VI) Circular Horizon: The view of the earth's surface as seen from a height is circular. With increase in altitude the circular horizon also widens. Had the earth's surface been flat the horizon would have been the same irrespective of altitude.

In modern days, all radio waves that travel over the earth's surface can be heard only within a certain horizon. Beyond this, the signal is not received. This is because of the spherical shape of the earth.

(VII) Sunrise and Sunset: The sun rises from the east and sets in the west. Sunrise and sunset occurs at different times at different places. This is because the earth rotates from west to east and therefore, people in the east can see the sun earlier than those in the west. Had the earth been flat, the sunrise and sunset would have occurred at the same time all over the world. (Fig. 1.6)

(VIII) Circumnavigation: Magellan's ship Victoria completed a round-the-world voyage in 1522. His ship returned to the same place from where it had started (Spain). It proved beyond doubt that the earth's surface is round (Fig. 1.7).

(IX) The Earth as a Heavenly Body: The earth belongs to the Solar system. All planets in this system are spherical in shape. It is because of their continuous rotation. So the earth being the member of the Solar system cannot be an exception.

Teacher's Note

When you watch an airplane take off, you can observe how the horizon changes shape as you gain altitude, demonstrating the earth's curvature in a way similar to the circular horizon evidence discussed in the text.

The Earth As A Planet

Earth, as a planet, is a spherical object in space that moves around the Sun (a star) and receives light from it. The planets do not have light of their own. They move around their stars in fixed paths called orbits.

Earth An Oblate Spheroid

The earth is said to be spherical; but it is not a perfect sphere. Its diameter varies at the Equator and at the Poles. It diameter at the Equator is 12,756 km while its Polar diameter is 12,714 km. This difference in diameter is due to the centrifugal force of earth's rotation at a great speed which forms a bulge at the Equator and a compression at the Poles. Thus, the earth is said to be an Oblate Spheroid. The shape of the earth is also described as Geoid, which means earth shaped.

Teacher's Note

The bulging at the equator due to Earth's rotation is similar to how a spinning ball of clay would bulge outward at its center - this is why tall mountains are easier to climb near the poles where Earth is slightly closer.

Earth As The Home Of Humankind

Our Earth is the only planet that supports life. Unlike other planets, it is covered with green vegetation, enormous blue-green oceans containing over a million islands, a large number of streams and rivers, huge land masses called continents with mountains, ice caps and deserts. Some form of life thrives virtually in every part of the earth - from the coldest part of the Poles to the warmest part of the Equator. Earth has certain features that make it habitable and thus, a unique planet. Some of these features are given below.

Distance From The Sun

The earth is at an optimum distance from the sun. Hence, it is neither too hot nor too cold.

The distance between the earth and the sun makes life possible on the earth. If it was closer to the sun, any life present would get burnt from too much heat of the sun. If it was farther from the sun life would freeze because of lack of heat.

Temperature

The earth is the third planet from the sun. It has an average temperature of 17°C which is suitable for life to exist.

If the average temperature on the earth's surface changes by only a few degrees, many species would perish due to extreme heat or cold.

The other two terrestrial planets, Mercury and Venus are very hot with maximum temperatures of more than 400°C. The remaining planets in the Solar system are very cold with surface temperature below 0°C.

Venus is the second planet from the sun but it is hotter than Mercury. This is because the atmosphere around Venus is mainly composed of carbon dioxide. The carbon dioxide produces Greenhouse effect on the surface of Venus. Thus, the temperature on its surface remain very high. The earth's atmosphere contains a very small quantity of carbon dioxide.

Atmosphere

Atmosphere is the layer of air around the earth. It is a mixture of gases like nitrogen (78%), oxygen (21%), carbon dioxide, helium and argon (less than 1%).

The atmosphere receives heat from the sun by solar radiation and loses heat by earth radiation. In this way a balance is maintained.

The earth's atmosphere is made up of life supporting gases like nitrogen, oxygen and carbon dioxide. Other gases include Helium and Argon.

Ozone present in the earth's atmosphere absorbs the harmful ultraviolet rays of the sun.

The atmosphere also prevents loss of heat from the earth's surface and helps to keep the earth warm.

Water

Earth is a watery planet with 70 per cent of the earth's surface being covered by water.

Distribution of water is responsible for moderating the climate and surface condition of the earth.

Teacher's Note

The conditions that make Earth habitable - moderate temperature, protective atmosphere, and abundant water - are the same ones that allow humans to thrive in diverse environments from deserts to rainforests.

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