Why

stuff goes into orbit:

Imagine I'm on the

moon, and I have a bunch of

tennis balls. If I throw a ball

parallel to the ground, but

not very quickly, it'll go for a while, but eventually

gravity will pull it to the ground and it'll stop. Now, as I throw the balls with greater and greater

velocity, they'll get farther and farther, but they'll all fall eventually. But then I throw one

really, really fast, and it never hits the ground! How can this be?

Let's examine the curved path of the ball as it travels any horizontal distance-- for simplicity's sake, 1

meter. Within that distance, it will fall away from a perfectly

horizontal path (the path it would take if there were no gravity) by a certain amount; let's call it d

_{vert}, and it increases as horizontal velocity increases, since it can travel farther before it hits the ground. Now, let's look at the planet we're on (in this case, the moon); more specifically, 1 meter of it. Since the planet isn't

flat (

last time I checked), it'll curve away from a perfectly flat distance by a certain amount within this 1-meter length.

Now here's the crazy part: If d

_{vert} (the amount the ball falls away from horizontal as it travels 1 meter) ever equals the amount that the planet curves away from the horizontal,

*the ball will never hit the ground*! Because the earth is curving away at the same rate that the ball is, it will go into orbit.