Mercury’s retrograde motion This video explains Mercury’s apparent retrograde motion in our skies: the inner planet appears to retrace its steps a few times per year. Every planet does this, every year. Retrograde motion is an optical illusion caused by differences in the planets’ orbital speeds. When a planet seems to reverse its direction in the sky, it’s called retrograde motion (from the Latin word retrogradus – "going backward"). Courtesy: Star Walk Based on video by James O'Donoghue / NASA