How do we know if the Earth is round?

Long long time ago, people thought that the Earth is flat. If you kept walking you will eventually reach the end off the world from where you can just fall off.

The first thing people needed to learn was that the Earth is spherical. Here's how we can deduce that Earth is spherical:

1. During a lunar eclipse we can see that the shadow cast by Earth on the Moon is a portion of a disk. This tells us that the Earth is round, but still some can have a doubt whether it is circular like a disk or spherical like a ball. Both the disk and sphere can have shadow like a disk. So how do we know that it is not circular like a disk but rather a sphere like a ball?

Semi-circular shadow of Earth on the Moon during the phases of a lunar eclipse

Only a sphere casts a circular shadow in every direction, whereas a circular disk casts an elliptical shadow in all directions apart from directly above and directly below.

2. The sailors found that when a ship is sailing away the first thing that disappears is the lower portions of the ship while the mast is still visible.  This could only happen if the earth was spherical. See the picture below:

When a ship is at the horizon, its lower part is obscured due to the sphericity of the Earth.

Still the dispute was not settled for a long time because people didn't know how it is possible to keep the ocean in a curved shape. Of course we now know that is is because of gravity. But back then people didn't know.

Once we found out that the Earth is sphere, someone needed to measure this size of the Earth. Well the Earth is so big how can we measure it's size?

You know who was the first person to measure the size of the Earth? It was Erathosthenes. In next email, I will tell you about how he measured the size of the Earth.