Mercury retrograde isn't what you think

Mercury retrograde happens several times a year. The planet's apparent change in motion doesn't affect Earth, but does caused visual deceit.

Mercury is a little-known, still mysterious world.

But one thing is certain. Mercury, the closest planet to the sun, can't actually travel backward in retrograde, and doesn't impact us on Earth. The notion of Mercury retrograde, a common reference in astrology wherein communication and travel are temporarily impaired, can, of course, be a fun musing. The event occurs when Mercury appears to start traveling the opposite direction in the sky.

But this is perceptive trickery. Mercury retrograde is apparent motion, such as when you pass a speeding car on the highway and it appears that car is moving backward. It's not an exceptional, or momentous, event.

"There's nothing unusual about it," Tansu Daylan, a physicist at Washington University in St. Louis who leads NASA telescope observations, told Mashable.

It happens so regularly — three or four times a year — that some people easily associate events (misunderstandings or travel woes) with the event. "People are actively searching for connections," Daylan said.

What causes Mercury retrograde

From our perspective, the planets, including Mercury, move west to east across the night sky, relative to the stars. Greek and Roman astronomers, in fact, referred to planets as "wandering stars."

If you, hypothetically, could look down on the sun from above, you would see the planets orbiting counterclockwise. This creates the easterly movement, from our perch on Earth. But this changes when two planets become aligned as they orbit the sun, Daylan emphasized. Then, an inner planet (Mercury) moves faster than an outer planet (like Earth), resulting in "apparent retrograde motion" — apparent because Mercury, speeding around our star at over 100,000 mph, does not suddenly start traveling backwards.

The NASA graphic below illustrates apparent retrograde motion, using Mars and Earth. A planet's appearance in the sky can change when planets align or one "overtakes" another, but the actual orbits don't change.

A depiction of apparent retrograde motion. Credit: NASA

Mercury's southern hemisphere, as captured by NASA's MESSENGER spacecraft. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury retrograde lasts for a few weeks at a time, so don't be surprised to hear how it's connected to a multitude of events. Tellingly, it most recently started again on Aug. 5, 2024 — after the global Crowdstrike disruption.

Though we know Mercury's orbit isn't doing anything strange in our solar system throughout the year, it will remain a world of mysteries. It's a planet that's difficult to observe from Earth, because of its proximity to the sun. "That means it can be spotted only briefly shortly before sunrise and just after sunset, and always appears close to the horizon," the European Space Agency explains. What's more, it's extremely challenging to reach — any spacecraft endeavoring there must contend with the sun's overpowering gravity. That's why Mercury is the least visited of the other inner planets. And, it's precarious to get too close to Mercury because the searing planet's surface, hot enough to melt lead, radiates heat into space. The ongoing BepiColombo mission is designed to resist these extremes as it makes unprecedented observations.

Planetary scientists hope to soon answer many questions about Mercury: Is there water? Is it geologically active? How did it get so close to the sun?

But, at least, its strange apparent motion is well understood.

"There's nothing fancy about it," Daylan said.