NORWAY — In a recent column, I expressed amazement that one side of our Moon always faces the Earth. Dawn Nelson, a professor of biology at the College of Southern Nevada, emailed me to say that while our Moon’s timing is amazing, it’s not unique.

“The phenomenon of the Moon facing Earth is well-known to astronomers,” she wrote, “and it’s not really bizarre at all. It’s called tidal locking.”

A little research on tidal locking left me gobsmacked. Earth’s moon is not the only one in our solar system that shows only one side to its planet.

Mars’ moon, Phobos, for example, shows one face to Mars the way our moon does to us. Deimos, Mars’ other moon, does the same.

Eight of Jupiter’s moons — Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, and Callisto — are tidally locked, so only show Jupiter one face.

And the list goes on. Fifteen of Saturn’s moons are tidally locked. Five of Uranus’s are. Two of Neptune’s. And if that’s not enough, Pluto is tidally locked to its moon, Charon, and Charon is tidally locked to Pluto, so they keep the same faces toward each other.

Pluto and Charon bring up an important point. Unlike those two, our Moon is tidally locked to the Earth, but the Earth is not tidally locked to the Moon. That means that though we always see the same side of the Moon, a person standing on the Moon will see all sides of the Earth as our planet rotates.

So what is tidal locking? It has to do with the gravitational force of an object in space (say, a planet) influencing the rotation rate of a satellite (say, a moon).

Spin a bicycle’s wheel and lightly stick a finger in the spokes. Your finger bumping against the spokes gradually slows the wheel down until it stops.

The gravitational force of a planet tugs on a moon and pulls it slightly out of shape. Instead of being perfectly round, the moon becomes somewhat of an oval. The bulgy part of the oval is like a finger brushing against the spokes of the planet’s gravitational force. But the moon is not a fixed object like your finger, it’s a small, spinning, orbiting orb. So every time the “finger” brushes against the “spokes,” the moon slows down a tiny bit until a certain balance is reached.

A better way to think of it is that every time the bulge spins by, the gravity of the planet tries to pull it back. This provides a braking action that eventually slows a moon’s rotation to the point that its rotational period matches its orbit and the bulge is permanently facing the planet. Our Moon, for example, takes 27 days to rotate on its axis and 27 days to orbit the Earth, which is why the same side always faces us. This is tidal locking.

Professor Nelson — as well as Professor David Batchelor, who unlike me is an actual astronomer — read a draft of this column and tidied up a few messes. Thank you both.


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