Here's a neat question you may have never thought to ask. How far away is the horizon?
Think about it. The Earth seems flat from our viewpoint, and if you are in a desert or out at sea you get a sense that the earth extends indefinitely in every direction. This isn't correct, though; the planet is round, so at some point the ground must drop away. When it drops below your angle of vision, you can't see it any more, and that's what we call the horizon.
So how far can you see before the planet curves away? If you're six feet tall, the answer is about three miles. Surprised? You aren't alone. Try asking a friend next time you're at the beach on a clear day to guess how far out across the ocean you are looking. She's more likely to say twenty or fifty miles than just three.
Where Is The Horizon Line?
Now that we're thinking about horizons, here's another cool thing to think about. The horizon is three miles away on earth. But elsewhere in the solar system it's closer or farther, depending on the size of the body you're standing on!
Take the moon. The moon is two thousand, one hundred and sixty miles in diameter, only about one quarter the size of the earth.
So the curvature at its surface is much more noticeable, and the horizon is much closer. It's only about 1.5 miles away.
One cool result? The surface bends so quickly on the moon that it's possible to stand inside some of the largest impact craters and not know you're in one. The walls of the crater are below the horizon on all sides!