Some 300 years after Pythagoras wrote about Earth being round, another Greek, Eratosthenes, calculated the Earth s radius. He used the distance between two cities and the angle of the sun s rays made at midsummer at noon. His measurement turned out to be 15 percent too large, but still impressive, considering it was done over 2600 years ago. One way you can see for yourself that Earth is round is by watching a lunar eclipse. A lunar eclipse is when Earth passes between the sun and the moon.
Earth then blocks the light of the sun and Earth casts a shadow on the moon.
If you watch as the shadow slowly passes over the moon, you will see that the shadow of Earth is as round as the Earth is.