|MadSci Network: Earth Sciences|
It's easy to wonder how we know the Earth is round. After all, it does look flat! But there is lots of evidence. Nowadays, it's very easy: look at a picture taken from space! All the pictures of the Earth show it to be a big ball. So why doesn't it look like it's round from the ground? It's because the Earth is so big that the curvature is very gentle. If the Earth were a kilometer across, it would be pretty obvious that it wasn't flat. If it were flat, the horizon would be exactly 90 degrees from the zenith, but that would be obviously untrue if the Earth were very small.
As it is, even from the ground you can see the curvature of the Earth. If the Earth were flat, the horizon would be infinitely far away, but it isn't. The horizon is actually only a few kilometers away (depending on your elevation). This is a classic way of showing the Earth is round: ships sailing over the horizon disappear from the bottom up, until only the tip of the highest part can be seen, and then that too sinks below the horizon. That can only happen on a round Earth.
It's a common misconception that ancient people thought the Earth was flat. Not true! Eratosthenes, thousands of years ago, showed not only that the Earth was round, but also derived a diameter for the Earth that was very nearly correct! He did this by measuring the length of a stick's shadow at two different latitudes. Because the Earth is round, the shadow lengths were different (one stick saw the Sun directly overhead, while the other saw it off to the side a bit). Since he knew the difference in the shadow lengths, and the distance between the two sticks, he was able to calculate the size of the spherical Earth.
Try the links in the MadSci Library for more information on Earth Sciences.