Planets are major bodies which orbit stars. They are not massive enough to ignite nuclear reactions at their cores like stars, yet they are massive enough in some cases to support gaseous atmospheres and complex chemistry. Since antiquity, five planets (from the Greek word for "wanderer") were known to move across the background of distant stars: Mercury, Venus, Mars, Jupiter, and Saturn. Early astronomers believed that the universe revolved around the Earth, and found it particularly difficult to describe the motions of the planets. Mars and Jupiter exhibited a puzzling behavior known as "retrograde motion" against the dome of the sky in which, on occasion, they seemed to reverse their direction over several days before returning to their original direction of motion. The ancient Greek astronomer Ptolemy was forced to invent a small circular motion, known as an "epicycle," to account for the variation of the mean motion...