From the October 2015 issue

Can you explain Mercury’s retrograde motion? Do other planets appear to do this also?

Robert Schneider, Merida, Mexico
By | Published: October 26, 2015
Mercury, as taken from the spacecraft MESSENGER.
When astronomers talk about retrograde motion, generally they refer to the apparent motion outer planets make as Earth passes them while orbiting the Sun. Most of the time, all the outer planets appear to move eastward through our sky. During retrograde motion, however, each appears to reverse direction and head westward. Note that this is an apparent motion. The same thing happens when you, in a car, pass another car going in the same direction. While you are passing that car (and because you are traveling faster), it appears to move in the opposite direction.

The two inner planets, Mercury and Venus, don’t exhibit retrograde motion for the same reason because they move faster than Earth. So, our planet never passes either of them. Some astronomers, however, define retrograde motion as any westward motion by a planet. For those that agree with that definition, even the inner planets retrograde as they move farther from the Sun (or the horizon) in the eastern morning sky or approach the Sun (horizon) in the western evening sky.

Michael E. Bakich
Senior Editor