Glossary

You may have heard that shape of the Earth is spherical. But are you sure that it is really spherical? The land beneath us looks quite flat. Think of at least one piece of evidence to show that the Earth is spherical. One such evidence, which was proposed more than 2300 years ago, is given in one of the later lessons. Of course, now we have photographs of the Earth taken from space, but we could fly the space-crafts only after knowing that the Earth is round!
 

If the Earth is spherical, why does it appear flat to us?

That is because it is too big! You may notice that a piece of a round flask looks almost flat.

Something similar thing is happening here. In Figure 1, as the circle becomes bigger, its rim appears more flat. Doesn't a portion of a football appear more flat than a portion of a ping- pong ball that is of the same size? You may know the rule in geometry that the curvature reduces as the radius increases.

Figure 1: Curvature decreases as the radius of the circle increases