Question

As viewed from Earth, why are the changing positions of Mars and the outer planets (against the background stars) sometimes characterized by distinct loop like and retrograde paths over time?

Answer 1

Typically, the planets shift slightly eastward from night to night, drifting slowly against the backdrop of stars. From time to time, however, they change direction. For a few months, they’ll head west before turning back around and resuming their easterly course. Their westward motion is called retrograde motion . The retrograde motion is an illusion caused by the motion of Earth and these planets around the sun.

As Earth passes the slower-moving outer planets like Jupiter or Mars or Saturn, for example, these more outward planets in orbit – which move more slowly than Earth in orbit – appear to reverse course in our sky for a couple of months.

A schematic of how retrograde motion works when Earth (T) passes an outer planet (P) as they both orbit the sun (S). The changing viewing angle from Earth makes the projection of the planet against the celestial sphere (A) move backwards (A2-A4) as we pass the slower planet.