Have you ever been warned not to make any decisions this week because Mercury is in retrograde? Or perhaps you heard a friend blame a misunderstanding with their partner on the retrograde motion of our planetary neighbor? Maybe you have had a string of unusually bad luck—your car breaks down, a package gets lost in the mail, your wallet gets stolen, and then you see the shirt you just bought go on sale the very next day—and you’ve thought, ‘surely the universe is out to get me this week!’
Mercury began a period of what’s known as “retrograde motion” starting on December 19th and will continue to be in retrograde through the end of 2016. Some people look to the motions of the planets to explain a series of unlucky mishaps or a seemingly random failure in communication that would ordinarily have gone smoothly. Although astrology, the practice of linking terrestrial events to celestial observations, has long been popular, there appears to be a growing trend of increasing concern and attention paid to the retrograde motion of the planets in particular.
It is of course human nature to wonder in the face of such unexpected problems: Why is this happening to me now? Did I make a mistake or was this unpreventable? But should we really plan our life choices around the timing and motions of the planets? What does it really mean for Mercury to be in retrograde?
Mercury: the Planet of Communication
Since Mercury is one of the five classical planets, along with Venus, Mars, Jupiter, and Saturn, visible from Earth and thus known in ancient times, it’s hard to know who discovered Mercury. Being the closest planet to the Sun, Mercury spends a lot of time within the glare of our star’s light, and thus was likely the last of the classical planets to be seen. Some of the first accounts of Mercury date back to the Sumerians in the 2nd millennium BC, but its name comes from the Romans who named the planet after the messenger god since it was only visible for short periods of time.
Mercury is thus known to be the planet of communication, travel, and contracts. Thus, those who believe there is a connection between the planet’s motion and events here on Earth often avoid signing contracts, having important conversations, or taking a big trip when Mercury is in retrograde.
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 Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein 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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