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Retrograde Motion

Retrograde might sound like a ‘70s album, but when we’re talking in astronomy terms, it’s something much more ‘far out.’

When referring to the planets, retrograde motion describes the apparent backwards motion (observed in comparison to the background stars) a planet takes in its week-to-week journey across the sky and it only occurs for a relatively short period of time.  This motion is also only seen in the outer (superior) planets: Mars, Jupiter, Saturn, Uranus, and Neptune. So what causes this apparent reversal in movement?

Retrograde motion of Mars in 2003 as seen from Earth.
© Eugene Alvin Villar, 2008

To answer this question, we need to remember that the planets in our solar system trace out an ellipse (not a circle) in their yearly orbit around the sun.  For most of the year, we on Earth see the outer planets moving across the sky in the same east to west direction as the stars do. This is called prograde motion.

Just like a race car driver on the inner lane passing a car on the outer lane, when the Earth passes an outer planet, it will appear to overtake it and that outer planet will look like it’s moving backwards across the sky to the viewer on Earth. Eventually, the outer planet will ‘catch back up’ and will continue its standard east to west motion (prograde) across the sky.

As Earth (blue) passes a superior planet, the superior planet (red) will temporarily appear to reverse its motion across the sky.
© Brian Brondel  (wikimedia commons)

During the month of April 2019, we have two planets (Jupiter and Saturn) that start their retrograde motion.  If you are an early riser, you can see these two planets a few hours before sunrise.