retrogradaatio

Retrogradaatio is a term primarily used in astronomy to describe the apparent backward motion of a celestial body in the sky. This phenomenon is not a true change in the object's direction of travel through space but rather an illusion caused by the relative motions of observer and observed.

The most common example of retrogradaatio is observed with planets in our solar system. As Earth, which orbits the Sun faster than outer planets like Mars or Jupiter, overtakes these slower-moving planets, they appear to temporarily reverse their direction of motion against the background stars. This apparent backward loop is a predictable consequence of orbital mechanics and Kepler's laws of planetary motion.

Ancient astronomers, unaware of the heliocentric model of the solar system, developed complex geocentric models to explain this perceived retrogradaatio. These models, such as Ptolemy's epicycles, involved planets moving in smaller circles (epicycles) on larger circles (deferents) around the Earth to account for the observed backward movement.

In modern astronomy, retrogradaatio is understood as a visual effect. The duration and extent of the retrogradaatio period for each planet vary depending on its orbital path and speed relative to Earth. While most commonly associated with planets, other celestial phenomena can also exhibit apparent retrogradaatio under specific circumstances.