The shape and size of the earth

The earth is a little speck in the universe, it is, to us, big in size. Its polar diameter is 12 718 kilometres, while its equatorial diameter, 42 kilometres longer, is 12 760 kilometres. The earth has an equatorial circumference of 40084 kilometres, polar circumference of 39 941 kilometres and a surface area of about 443 million square kilometres. This is how large the earth is.
Many years ago, people thought that the earth was flat in shape. Later, they said it was circular. But none of these defines the true shape of the earth. To get a good idea of the shape of the earth.

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The image of the earth shows that it flattens at the north and south ends or the poles and bulges out at the middle or the equator. That is why the equatorial diameter is slightly longer than the polar diameter. This shape is called oblate spheroidal, and any object of that shape is called a geoid. So the earth is a geoid, though ordinarily it looks spherical from afar.

Proofs of the earth`s oblate spheroidal include the following:

• Day and night. The fact that the sun shine over the whole earth at a time and some part hidden behind it, proves this shape. That is, as the earth rotates, the sun rises over the area it will shine for the day and sets over the area that will have darkness for the night. This shows that the earth is spherical in shape.
  
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• Ship's increasing visibility. When a ship approaches the shore, the observer does not see the whole ship all at once. He sees the smoke from the chimney first, then the chimney, next the hull and finally the entire ship. This is because the surface of the sea is curved. If it were flat, the entire ship would be visible all at once.

• Lunar eclipse. Lunar eclipse or eclipse of the moon, occurs when the sun shines on the earth and the shadow of the earth is thrown on the moon to Cover the entire moon or part of it. The edge of this shadow is an arc of a circle. Since only spherical or circular objects can cast circular shadows, the earth must therefore be spherical in shape.
  
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• Circumnavigation of the earth. One of the earliest proofs, however, is that sailors have round the earth in various directions, sailed and have come back to where they started without falling off any edge! Sir Francis Drake was one such early sailor who did., this. it is done more frequently nowadays. If the earth were fiat, the navigators would have fallen off its edge! This having not happened, the earth must then be spherical in shape.

• Constant circular nature of the retreating horizon. The horizon is where the sky appears to touch the ground. On a sea or in a desert free from obstructions, this horizon is circular. As one goes up, the horizon recedes, becomes bigger, but till remains a circle. Since only a spherical object can have a horizon that behaves this way, the earth must therefore be spherical in shape.

• Constant value of gravity at sea level. Again, the force of gravity varies with distance away from the centre of the earth. However, this force is constant at sea level all over the globe. So the sea level at all parts of the earth must be equidistant from the centre of the earth. This means that the earth must be spherical in shape.

• Satellite photographs of the earth. Satellite photographs of the earth taken from space by astronauts on Apollo 8 and Apollo 11 spacecraft respectively, show the circular outline of the earth. What more proof do we then require than photographs to show that the earth is spherical in shape.
  
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• As a matter of fact, if we want to go by analogy, all the other heavenly bodies as we see them are round; why then should the earth, also a heavenly body, be different?

Conclusion

With all this mass of evidence available to us the fact that the earth is oblate spheroidal in, shape (or spherical as seen from afar) can no longer be in doubt.

Reference

https://www.physics.queensu.ca/~hanes/ASTR101-Fall2016/PPTS/Theme2-Part3.pdf

National Geographic

Wikipedia