Any keen observer of the night sky soon realizes that a handful of objects behaves differently from all the rest. While most of the points of light remain fixed relative to one another, returning to the same spot year after year, a few planets (from the Greek word for “wanderer”) move about in complex ways. Most of the time they travel from west to east relative to the fixed stars, but occasionally they stop and then reverse direction for a couple of months before resuming their normal eastward motion.
Understanding how the planets move was the central question in cosmology in the days when the planets were thought to make up most of the known universe.
The task of explaining these odd movements fell to Nicolas Copernicus and Johannes Kepler. Copernicus correctly deduced that the Sun lay at the center of the solar system. Like a faster runner viewing his competitors, Earth sees a slower outer planet appear to back up as we get ready to “lap” it and then return to its normal direction of motion. Kepler refined this picture by showing that all the planets move in elliptical orbits and at speeds that vary with their distance from the Sun.