From losing your wallet in the subway to a newly-broken part in your car, the planet closest to the Sun is often blamed for life's misfortunes, big and small. You may hear responsibility placed on "Mercury retrograde (水星逆行), referring to how the planet appears to move backward in its march across our sky fora few weeks about every four months. The apparent backward movement of Mercury is an actual astronomical phenomenon, but its connection to happenings on Earth is largely denied as pseudoscience.
Still, the idea that Mercury holds power over our communication is popular in the West. Retrograde motion has captured sky-gazing humans' attention for millennia. Among ancient astronomers' Earth-centric ideas about the heavens, a celestial body appearing to move backward would've presented an eye-catching sight. Indeed, the apparent retrograde motion of planets was documented quite early in human history. Mercury retrograde was probably first documented by Babylonian astronomers around the 7th century B.C., says Mathieu Ossende, a historian.
The idea that the positions and motion of celestial bodies could predict the fates of a nation, a ruler, a harvest, or an event endured into medieval Europe. "You'd have someone really powerful go to an astrologer and say, ‘I want to surround a castle or attack my enemies, what's a good time?'" says Nicholas Campion, an expert of cosmology culture. Mercury in retrograde was believed to prevent such endeavors, as well as these astrologers' divinations (占卜). In fact, Campion adds, some astrologers might say they couldn't read the chart when Mercury was in retrograde.
Mercury isn't actually moving backward in its orbit around the Sun. Retrograde motion is an optical illusion caused by all the planets moving at different speeds from one another. It's kind of like driving on a highway with multiple lanes going the same direction, explains Carolyn Ernst, a planetary scientist. If you're passing a car in another lane that's going more slowly, it might appear to move backward compared to you, despite going in the same direction, and vice versa. That's what happens when Mercury laps Earth.