# Shape and Size of the Earth

## During earlier times it was believed that the shape of the Earth is a flat disc. The main believers of this idea were the Greeks, the Babylonians and, the ancient Indians. Aryabhatta, the famous Indian astronomer believed that Earth was spherical in shape and rotated on its axis. Greek philosophers like Aristotle and Pythagoras argued that the Earth was a sphere. Sir Isaac Newton suggested that the Earth was not a perfect sphere, but rather somewhat flattened at its poles and bulged at equator. There are various evidences to prove that the Earth is not flat. One of the evidence is that places in the east see the rising sun first. The varying height of Polaris at different latitudes also proves that the Earth is not flat. If the Earth was flat, then the height of Polaris would have been same at different latitudes. The Earth is a sphere. This is proven by the circular shadow of Earth on Moon’s surface during a lunar eclipse. Circumnavigation also proves that the Earth is a sphere. Angle of Pole star is always observed at 90° at the North Pole. Shape of heavenly bodies like the Sun, Moon and other planets are seen as spherical. The Earth is no exception, as it is also a planet. The Bedford Level Experiment was conducted by Rowbotham between 1838 and 1870. This experiment clearly demonstrated the curvature of the Earth. The existence of different time zones on the Earth also strengthens the fact that Earth is spherical in shape. Earth’s photograph taken from space clearly shows its spherical shape. The Earth is an oblate spheroid. The Earth is not a true sphere. It is compressed at the poles and has a bulge at the equator. One of the evidences of Earth being ‘Oblate Spheroid’ is Equatorial diameter is greater than the polar diameter. A cross-section through the poles is an ellipse rather than a circle. During the expedition to Cayenne, Jean Richer observed that his clock was losing two and a half minutes per day. This is due to the shape of the Earth. Gravity measurements also prove that Earth is oblate spheroid. Gravity causes weight in the objects. Equatorial circumference of the Earth is greater than polar circumference. The circumference of the Earth was first measured by Eratosthenes, a Greek astronomer. According to Eratosthenes’ calculation, the circumference of the Earth is equal to 40,000 km. This calculation is very close to modern calculations. A Great Circle is created whenever a sphere is divided exactly in half through its centre. It is the largest possible circle drawn on the surface of a sphere. A course of travel along the great circle is called great circle route. It is used in navigation. A small circle is any circle formed by planes passing through a sphere anywhere except the center.

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• Q1

How is the mass of the Earth calculated?

Marks:1

Using Newton’s law of gravitation, mass of the Earth is calculated from the orbit of the Moon.

• Q2

Define centrifugal force.

Marks:1

Centrifugal force is the apparent outward force on a rotational body, directed away from the centre.

• Q3

What is known as a great circle with reference to the Earth?

Marks:1

Equator is known as a great circle with reference to the Earth.

• Q4

Who calculated the circumference of the Earth?

Marks:1

The circumference of the Earth was first calculated by Eratosthenes in 200 B.C.