The science behind the Mercury retrograde

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Mercury retrograde occurs three to four times a year when Mercury appears to temporarily change the direction of its orbit. This is actually happening right now, from Jul. 7 to Jul. 31.

In astrology, Mercury governs communication, travel, and learning. Astrologers believe that Mercury retrograde affects life here on Earth, specifically, communication and technology. For this reason, Mercury retrograde is often blamed for miscommunication, technological bugs, failed business deals, missed flights, car troubles and even cell phones breaking. But there is no concrete science to back that up.

Aside from astrology, planetary retrograding is a real astronomical phenomenon. To explain it, let’s begin with the basic layout of the solar system: Mercury is the planet closest to the sun and Earth is the third closest, after Venus. Planets closer to the sun move more quickly in their orbit and planets that are farther away move more slowly, but they all move in the same direction.

When planets retrograde in the sky they aren’t actually changing directions; this is just what they appear to be doing when observed in the sky from Earth. It’s an illusion that unfolds over several weeks or months. This illusion happens when Earth laps an outer planet, like we do to Mars every two years, or when an inner planet laps Earth, like Mercury does every four months.

As Mercury passes by us, our line of sight shifts, so Mercury will appear to loop back on itself for about three weeks, before resuming its normal orbital direction. It’s simply a function of two planets orbiting in the same direction at different speeds. If you were standing on Mercury, Earth would appear to retrograde every so often as well. The key here is to remember that Earth is moving too.

It’s easy to see how baffling planetary retrograde must have been to early astronomers who thought Earth was stationary and at the centre of the solar system. In the second Century, Claudius Ptolomy developed a model for planetary motion full of complicated loops and orbits within orbits to account for retrograding. Despite being completely false, that model was widely accepted for 1,500 years because it made sense based on the knowledge of planetary motion that was available at the time. Once Johannes Kepler demonstrated that all the planets, including Earth, move around the sun, retrograde motion was easily explained as simply an illusion. Most of the time in nature, the simplest explanation is the right one.

If there’s anything we can take away from explaining the Mercury retrograde phenomenon, it is that we may be vulnerable to many illusions, and we can develop false pretenses when we think that everything revolves around us.