From the July 2020 issue

What does it mean for Venus and Mercury to be in retrograde?

By | Published: July 31, 2020 | Last updated on May 18, 2023
When an inferior planet, such as Venus, passes Earth, differences in the planets’ orbits make the planet appear to stop and then move backward, or retrograde, in the sky for a brief time. This chart shows Venus’ path between April 1 and July 10, 2020, as it appears to backtrack against the background stars for about 40 days.
Astronomy: Roen Kelly
Q: On page 51 of the February issue, there is a picture claiming to show Venus in retrograde. I have believed and taught for 40 years that retrograde motion only occurs with the planets beyond Earth’s orbit. So please explain how Venus could be in retrograde, or was this a mistake?

John Shonle 
Amherst, New Hampshire 

A: The apparent retrograde motion of planets (and other objects) on the sky is an illusion caused by the fact that objects in our solar system orbit the Sun at different distances and speeds. This is certainly easiest to picture for superior planets — those outside of Earth’s orbit — such as Mars. Earth circles the Sun every 365 days; Mars takes 687 Earth days to do the same. Our planet has a shorter path to travel, so at some point, we “catch up” to Mars and then pass it. Just as passing a slower-moving car on the highway makes that car artificially appear to move backward from your point of view, Mars appears to move backward, or retrograde, relative to the background stars for a period of time. Once your car (or our planet) has pulled far enough ahead, the retrograde motion disappears.

But as your question brings up, can this happen with the inferior planets Venus and Mercury? The answer is still yes, these planets do exhibit retrograde motion. Their retrograde motion occurs because they circle the Sun much faster than Earth and sometimes overtake our planet as they swing around our star. That same effect causes them to first pause, then move “backward” (or westward) relative to the background stars, before pausing and resuming their eastward motion.

Alison Klesman 
Senior Associate Editor