retrogrados

Retrogrados are the apparent backwards motion observed for some planets when viewed from Earth. The phenomenon occurs when Earth, moving faster in its orbit, overtakes and passes an outer planet or when a faster inner planet overtakes a slower outer one. During a retrograde period the planet appears to move westward across the sky, opposite to its usual eastward direction relative to the stars.

The first systematic observations of retrogrades were recorded by ancient astronomers such as the Babylonians, who logged the unusual paths of Mars and other planets. The Greeks later interpreted these motions, with Aristarchus and Ptolemy developing geometrical models that explained retrogrades through epicycles and deferents. In the 16th century, Copernicus’ heliocentric model clarified that retrograde motion is an optical illusion caused by the relative motions of Earth and the other planet, instead of an intrinsic change in planetary direction.

Retrogrados of outer planets such as Mars, Jupiter, and Saturn last up to several months and can be forecasted precisely because the motions of the planets are well understood. Retrogrades are important for astronomers because they indicate the relative positions of planets in their orbits, and historically they were significant events in navigational calculations. The term comes from the Latin retrogradus, meaning “moving backward.”