retrograde

Retrograde refers to the apparent backward movement of a planet across the sky as seen from Earth. This occurs because Earth and the other planet orbit the Sun at different speeds; when Earth overtakes or is overtaken by another planet, that planet appears to move in reverse against the background stars for a period of time. This optical illusion is temporary and only an observational perspective. Retrograde motion is a normal part of planetary movement and helps astronomers study the orbits and positions of planets within our solar system.