https://askanearthspacescientist.asu.edu/top-question/flat-earth
How Do We Know the Earth Is Round?
Patrick McGurrin
When you are able to get up, above the horizon, whether in a plane or in the International Space Station, you can see that the edge of Earth is rounded. By watching as the Earth also rotates from that view, you can see that the Earth is a rotating sphere and is not flat. Image by NASA.
Like the other planets, the Earth is a sphere. It rotates around its own axis slowly but continuously, and completes this rotation about once every 24 hours. As the Earth rotates, part of the Earth is facing the sun, and part of it faces away. This is why some parts of the world have day and night at different times
There are a number of reasons why scientists know that the Earth is round, and not flat. It turns out that we can test many of these reasons out for ourselves, too.
- If the Earth was flat, we would be able to look out and see very far, assuming that there were no buildings in the way. We could see another city that’s hundreds of miles away. Or a ship sailing out to sea. The boat would stay in view no matter how far out to sea it went.
However, if we look out when standing on the ground, we can’t see that far. We can’t look out into the distance and see a town that’s miles away. And a ship will slowly start to disappear from view as it gets farther away. This is because the Earth is round. Things in the distance are not visible because there is a curve to the landscape.
One way to test this is to watch the sunset. Find a spot where you can see the sunset, and also have a place close by where you can safely move higher, such as a tree or building. Or even start laying down and then stand up. On the ground, the sun will eventually disappear beyond the horizon. When this happens, quickly go to your higher spot. From there, you’ll be able to see the sun again for a brief time.
- You have probably seen that when the sun is out, it causes a shadow to form if it hits something. It turns out that we can use shadows as another piece of evidence that the Earth is round.
The fact that the sun sets at different times in different locations lets us know that the Earth is a sphere. Image by NASA.