Over a single night, the planets behave much like the stars; they appear to rise in the east and set in the west. However, over the course of many nights, one will recognize that the movement of planets among the stars is quite intricate. The speeds and brightnesses of the planets fluctuate significantly, and while they typically travel eastward through the zodiac, they will periodically reverse course and move westward through the stellar background. This phenomenon is called apparent retrograde motion, and these periods can last anywhere from a few weeks to a few months.

For ancient astronomers who believed in a geocentric universe, this presented a problem. If planets supposedly moved in perfect circles around a stationary Earth, then what could be causing this peculiar backward motion? Greek astronomers like Ptolemy suggested that each planet traveled around Earth on a small circle, or epicycle, that simultaneously moved upon a larger circle, or deferent.

Apparent retrograde motion can be explained much more simply with a heliocentric universe. Each of the planets orbits the Sun at a different rate; Mercury and Venus have shorter orbital periods than Earth since they are closer to the Sun, but Mars and the gas giants take a longer time to complete their revolutions. As the Earth passes or is passed by another planet in its orbit, the other planet appears to move back and forth relative to the stars in the distance. We know today that the heliocentric theory is the right one, but it would take almost 2,000 years from the time it was first suggested by Greek astronomer Aristarchus in 260 B.C. to be widely accepted. Nevertheless, the complexities of planetary motion would spur much of the debate over our planet’s place in the cosmos.