What Are the Forces That Keep All Planets in Orbit?

I'm Eric Loberg, director of the Taylor Planetarium at the Museum of the Rockies. I'm going to discuss which forces keep an object in orbit. An orbit is when one object circles around another object. And so that could be a moon going around a planet. Or that could be our planet earth going around the sun. It doesn't really matter, all the forces remain the same. There are two major forces that keep these in orbit. One of the forces is gravity. Gravity is always pulling on everything. If something has a bigger mass it pulls more. Gravity makes the smaller object fall into a bigger object. The bigger object has more mass the small object is trying to fall into it all the time. And so gravity is pulling the smaller object be it the earth falling into the sun. Or the moon falling into the earth. There's another force though and that's momentum. Momentum is always keeping these objects on their path. And so our moon is constantly on the path around the earth. Or the earth is constantly on the path around the sun. These two forces have to remain in perfect balance. If anything changes the other force has to change as well. This is Newton's first law. Any object that tends to go in motion has to stay in motion as well. And so we have two motions. We have the orbit that's going all the way around. And gravity that's pulling it in. If either of these changes the other would change as well. So lets say our big planet simply disappeared. Well that moon all of a sudden doesn't have a gravitational pull and it would just shoot off into space. Or lets say that the moon suddenly stops. All of a sudden it's momentum around the planet or the sun would stop and that moon would shift and fall into either the planet or the sun. It fall into the larger object of mass. And so the two forces together combined are what keeps that planet in orbit. My name is Eric Loberg with the Taylor Planetarium at the Museum of the Rockies.