Variously called Newton's Cannon or Newton's Mountain, This is an animation of a thought experiment by Isaac Newton.
He wrote about firing a cannon horizontally from the top of a high mountain. If the speed of the projectile is too low, the projectile will hit the ground. However, if the speed is high enough, the projectile will continue to move parallel to the gound as it curves out from under it. The idea is that the projectile will orbit the earth until it comes back around and hits the person who fired the cannon in the back of the head. Thanks to Daniel V. Schroeder, Department of Physics, Weber State University, Ogden, UT, who developed this particular animation in HTML5, so that it does not require installing any other software.