Question

How can we prove that the Earth is a sphere?

Answer 1

We go to space and view the earth from a distance :)

Answer 2

shadow cast by earth on moon is spherical.
Earatosthenes measured the circumfrence of the Earth. On June 21 2200 years ago. In Alexandria Egypt. He had to make two measurements of the angles made by shadows at two different locations. Syene and Alexandria. If the Earth was flat.Then, there would be no shadows cast by both sticks at different locations. Given that the Sun is overhead at noon. But, there was a shadow cast at Alexandria at noon. While there was no shadow cast at Syene. That can only happen to a planet if the shape is round. So a helper helped him measure the distance between Alexandria and Syene (800km). From the shadow lenght in Alexandria,the angle A can be measured. But,from simple geometry if two parallel straight lines are transected by a third line,the alternate interior angles are equal. Angle B equals angle A. So,by measuring the shadow lenght in Alexandria,Eratosthenes concluded that Syene was A = B = 7 (seven degrees is (7deg/360 deg) or1/50th the circumfrence of the Earth ) away on the circumfrence of the Earth. Eight hundred kilometers * 50 is 40,000 km. So,that's the circumfrence of the Earth.He was the first person to measure a planet.

Source: Cosmos by carl sagan

Answer 3

hmm he measured the circumference already ASSUMING it was a sphere. That doesn't prove it is a sphere. But good answer nonetheless, I've been meaning to watch the series [Cosmos].

Answer 4

obseravtion and experimentation reached in same results, it means thought experiment was success. thats the common method we follow in science

Answer 5

I'm not saying your answer doesn't validate the earth is round, but it doesn't prove it is explicitly, since the same argument could be made for a cylindrical earth. Contrary to what you may think real science rarely involves thought experiments; armchair scientists are just a step above philosophers.