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What Does "Mercury in Retrograde" Mean?

You've probably heard the term 'Mercury in Retrograde," before, often used to explain bad luck or unexpected problems. But what does it really mean for Mercury to be in retrograde?

By
Sabrina Stierwalt, PhD
5-minute read
Episode #219

What Is Planetary Retrograde Motion?

First and foremost, it is important to point out that despite the name “retrograde motion,” there is no difference in the actual motion of the planets while the planet is in retrograde. A planet never stops in its tracks and begins orbiting backwards. Whether a planet is in retrograde depends only on our perspective and how that planet’s motion appears from Earth. So when Mercury is in retrograde, the difference in the planet’s motion is only an apparent one and not a real difference for anyone who might be, say, living on Mercury.  

So what causes this difference in perspective? Since the Earth moves around the Sun while it rotates about its own axis, we view the same spot on the sky four minutes earlier each night. In other words, the stars you see directly overhead at midnight tonight will be in that same spot directly overhead at 11:56pm tomorrow night because the Earth will have moved a little further along in its orbit around our star. Most objects in the sky follow this eastward motion, known as prograde motion.

However, as we’ve discussed in relation to the planet Mars, there are periods of time when planets appear to move backwards in the opposite direction, movement known as retrograde motion.  The actual geometry differs slightly depending on whether or not the planet is closer to the Sun than the Earth or farther away but always results from the fact that planets move in their orbits at different speeds.

Mercury’s orbit around the Sun is smaller than our planet’s orbit, and Mercury moves faster in that orbit than the Earth does in our orbit. So, at some point, Mercury will catch up to the Earth’s orbit and overtake us causing it to appear as if Mercury is moving backwards. In fact, this happens 3-4 times each year.

You can recreate this retrograde motion for yourself to see how it works. Imagine that you are the Earth and stand facing a circular track while you watch someone else (i.e. Mercury) run around the track. As the runner passes you, you will see her move in one direction (from left to right, for example). But as she turns the corner and continues to run, she will appear to turn back and head in the opposite direction across your view. In reality, Earth is not stationary, so if you want this analogy to be more accurate, you can slink slowly along the sidelines as you watch.

Does Mercury’s Retrograde Motion Affect Us on Earth?

History has given us some pretty significant disasters during periods when Mercury was in retrograde including the Titanic sinking in 1912, the Hindenburg explosion in 1937, and the stock market crash of 1998. However, Mercury is in retrograde ~20% of the time so it’s no surprise, statistically speaking, that historically bad events occurred during one of these periods.

The motions of the planets are also highly unlikely to affect our day-to-day activities based on their gravitational pull. Given the proximity of the Moon, even though it is less massive, the gravity you feel from the Moon is far larger. And, of course, the gravitational pull of the Sun dominates our planet’s position in the solar system. Even the gravitational pull you feel from someone moving next to you would be stronger than the shift in Mercury’s gravity as it moves between prograde and retrograde motion.

So if it’s not Mercury’s apparent motion on the sky causing us bad luck, then what is it? Usually the answer comes down to statistics. Is it unusual for a series of mishaps and miscommunications to happen all at once? Sure, but that doesn’t mean that it’s impossible. So it is this astronomer’s opinion that you should go ahead and have those important conversations and take those exciting trips no matter what time of year it is.

What to know more about Mercury? Check out NASA’s page of basic Mercury facts. Think you already know a lot? Check out this quiz over at livescience.com

Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com

Image of Mercury courtesy of nasa.gov

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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.