Key Takeaways:
- Retrograde motion, typically observed in outer planets, is an apparent westward movement resulting from Earth's faster orbital speed overtaking those planets.
- This apparent reversal of direction is analogous to observing a slower vehicle appearing to move backward when overtaken by a faster one.
- Inner planets (Mercury and Venus) do not exhibit retrograde motion in the traditional sense because Earth's orbital speed is slower than theirs, preventing overtaking.
- An alternative definition of retrograde motion encompasses any westward movement of a planet, including the apparent westward movement of Mercury and Venus relative to the Sun or horizon.
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
Senior Editor
