What direction does retrograde motion refer to?

Retrograde motion refers to the apparent movement of a planet in the opposite direction to that of other celestial bodies in the solar system. In the context of planets as observed from Earth, this means that during certain periods, a planet appears to move from east to west across the sky, which is opposite to the usual motion of celestial objects that typically move from west to east due to the rotation of the Earth.

This retrograde motion can occur as a result of various factors, such as the relative positions and motions of Earth and the other planet involved. For instance, when Earth passes by an outer planet like Mars, Mars can appear to move backwards in the sky for a short period. This is a key concept in understanding how we perceive the motion of planets and why retrograde motion is observed.

In contrast, other directions mentioned in the options (such as north to south and south to north) do not pertain to the general understanding of retrograde motion in the context of celestial observations. Retrograde motion is specifically characterized by this east-to-west directional shift, making it a vital concept in the study of planetary motion and the dynamics of our solar system.