Sir Isaac Newton was an English mathematician, chemist, astronomer, theologian and author generally regarded as one of the most prominent scientists of all time and a central figure in the science revolution. He is credited with developing classical mechanics and was an influential influence in optics and was responsible for discovering the infinitesimal calculus.

Despite vastly predating any of our space travel, he also developed what may be the best way of explaining orbital mechanics. In other words, how does the International Space Station just stay above the earth without coming crashing down? How does the moon orbit the earth and why does it need to go in circles around it?

Newton's explanation uses a theoretical cannon that we imagine is perched up high on a mountain on Earth. If you were to drop the cannonball from that same height, it would just fall down to earth.

That's not quite an orbit, but we see something familiar when we load up that cannonball and shoot it outwards.

The ball takes the same amount of time to fall but due to the cannon's force, it makes this nice arc starting from the top of the mountain and ending at the surface of the Earth.

Newton's "aha" comes from when we shoot this cannon with so much force, this arc stretches out to the point where the arc continuously follows the curvature of the Earth.

This of course would only be the case if there were no air resistance to slow down the ball. A mountain that could reach outside of our atmosphere would do the trick.

We can also see how an elliptical orbit could be achieved with even greater force.

Another way of thinking of orbital mechanics: put the cannon ball on a string and swing it in a circle above your head. Notice how the centripetal force on the ball caused by the string pulling it towards the center is counteracted by the linear momentum of the ball. (Let go of the string and you'll see this linear momentum). In other words, the competing forces cause the ball to orbit around your hand.

Gifs credit: CC BY-SA 3.0 Todd Timberlake author of Easy Java Simulation, Francisco Esquembre