How planets move is one of the earliest questions that ancient scientists grappled with in trying to determine the rules of the universe. Early theories postulated that Earth was the center of the universe, and all celestial objects orbited around it. With Galileo's findings, it was revealed that the sun, not the Earth, was the center of our solar system, and the planets moved around it a varying speeds and angles. Today's theories of planetary motion are based on the work of 16th century German astronomer Johannes Kepler.
Using the work of his mentor, Tycho Brahe, as a basis for his theories, Kepler changed the worlds of astronomy and physics through his three laws of planetary motion. Although at the time only six planets were known, his theories were confirmed more than a century later by Newton, and have held up well for over 400 years. Although his theories are somewhat perplexing to the non-astronomer, they greatly changed the playing field for the world of planetary science.
The first law that Kepler determined was that planetary motion is elliptical rather than cyclical. Rather than moving in a circular pattern around the sun, each planet moves in an oval-shaped orbit. This law was in complete disagreement with the prevailing theories of planetary motion that had existed since the time of Aristotle, but overwhelming scientific evidence eventually proved Kepler's new theory to be true.
Kepler's second law deals with the speed that planets move while following their orbit. Planets change speed relative to their position to the sun; when they are closer they speed up, and when they are farther away they slow down. Kepler's second law states that over equal periods of time, a planet will move an equal distance. Basically, the distance it would travel in one month is longer but at a higher speed when close to the sun, while far from the sun it would move slower but have less distance to cover. According to this law of planetary motion, the speed balances out the distance, so a planet will almost always cover the same amount of distance in a given period of time.
The third law of planetary motion that Kepler divined is more mathematical and complicated in nature. While the first two laws deal with how a planet moves relative to the sun, the third law compares a planet's movements against other planets. Basically stated, if you square the amount of time a planet takes to complete an orbit, and divide it by the cubed average distance of the planet to the sun, you will come up with a near identical ratio for every planet. This means that the orbiting time of a planet is directly proportional to how large the orbit is, so the ratio is almost exactly the same no matter what planet is being described.
Planetary motion helps describe the rules of the solar system, but its usefulness does not end there. In addition to explaining how the planets move, it also helps modern scientists determine the orbiting patterns of satellites and other man made objects put into space. Kepler's laws also have helped to explain the orbiting pattern of new planets just being discovered by advanced technology, even if we cannot visually observe them.