By Elizabeth Douglas, Marketing and Communications
If you’re interested in astronomy, astrology or have spent any time on social media, you’ve likely heard the term “Mercury in retrograde.” When life becomes challenging, mistakes happen or it feels like things just keep going wrong, it has become commonplace to blame them on our solar system’s smallest planet. And while turning to the stars for answers may provide a certain level of comfort or entertainment, we at Douglas want to know what’s physically happening in the sky above us. We caught up with Physics instructor Dr. Will Gunton to break down the real meaning of “Mercury in retrograde.”
What does “Mercury in retrograde” actually mean?
Retrograde motion means that Mercury appears to be moving backwards. If you track the position of Mercury or any planet over many weeks, you will notice that the planet slowly moves from the west to east relative to the stars. This is a normal motion due to the orbit of the planets around the Sun.
However, occasionally a planet will appear to stop and turn around, moving backwards – now from east to west relative to the stars – for several weeks before turning around again and continuing the normal west to east motion.
So, when Mercury is in retrograde, it just means that it appears to be moving backwards compared to its normal motion relative to the background stars.
Why does it appear to be moving backwards?
This is an illusion caused by Earth and other planets “passing” each other during their orbit around the Sun. This is a similar effect to when you pass a car on a highway, and it looks like the other car is moving backwards.
Is Mercury’s retrograde motion different than other planets?
Yes. For superior planets – those that are further away from the Sun than the Earth – retrograde motion occurs when the Earth passes them.
However, Mercury is an inferior planet, which means that it is closer to the Sun than it is to the Earth. Because of this, Mercury orbits the Sun more quickly than the Earth does, thus catching up to and passing the Earth. Mercury’s distance from the Sun means that it’s motion relative to the background stars is dominated by the Sun’s west-to-east motion. However, when Mercury catches up to and passes the Earth it is on the same side of the Sun as the Earth, and its orbital motion is able to overcome the eastward motion of the Sun, making Mercury appear to move from east to west, which is retrograde (or backwards) motion.
How often does Mercury enter retrograde?
Because Mercury orbits the Sun every 88 days, Mercury and the Earth “pass” each other in orbit three or four times a year, which means Mercury undergoes retrograde motion most often of all the planets.
Don’t be alarmed when Mercury goes into retrograde starting Nov. 24. Instead, look up at the stars and ponder the orbits and influence of the planets in our solar system!
Visit our website to learn more about Physics and Astronomy at Douglas College.