I'm reading Cosmos by Carl Sagan. It's pretty sweet.
Kepler, this guy that lived a while ago, decided to try to figure out the orbits before anyone knew that they existed. He sought out to answer, in essence the questions about the movement of the planets. How do they go? And in order to find the answer to this question, he made all of these calculations assuming that the orbits were circular and was 8 minutes off in the end. He writes that all of his findings amounted to "only a single cartful of dung" because of these stinking eight minutes. Then he figured out that the orbits weren't perfect. Why should they be perfect? Nothing on earth is perfect. The orbit of Mars wasn't a perfect circle, rather, an ellipse. "Ah, what a foolish bird I have been!"
So when the planets are close to the sun, they are moving really fast, because the gravity of the sun is stronger there. When they're far away, they move slower. And this is described in Sagan's book with the words "forever falling toward, but never reaching, the Sun."
Kepler discovered that the heavens function in a manner that science could touch. "Astronomy is part of physics."
Check this out: "Kepler believed that the speed of each planet corresponds to certain notes in the Latinate musical popular in his day - do, re, mi, fa, so, la, ti do. He claimed that in the harmony of the spheres, the tones of Earth are fa and mi, that the Earth is forever humming fa and mi, and that they stand in a straightforward way for the Latin word for famine. He argued, not unsuccessfully, that the Earth was best described by that single doleful word."
Another quote by Kepler: "Do not sentence me completely to the treadmill of mathematical calculations - lave me time for philosophical speculations, my sole delight."