* 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 /

Planets usually move across our night sky from west to east relative to the background stars. This steady motion is a result of their orbits around the Sun. But every so often, a planet like Mars appears to hit the brakes, reverse direction, and move backward - a puzzling motion known as retrograde. This animation breaks down the mystery behind retrograde motion, focusing specifically on Mars. You'll see how the motion of Earth and Mars in their respective orbits creates the illusion of Mars loo