The Other Side of the Moon

Why does the moon show only one face to the Earth?

Why can't we see the far side of the moon? Lunar satellites have sent pictures, but only Apollo astronauts have seen the back of the moon in person. (SpaceX is selling tourist tickets for a future fly over.) China landed their Chang’e-4 probe there—the first "boots on the ground" in that unknown territory. What has kept the back side hidden?

Gravity—gravity wins in the end.  The moon no doubt spun on its axis when it first hooked up with Earth. Over billions of years, our planet's greater gravity brought the moon's spin into synch with its orbit. It was a gradual process. Earth's gravity distorted the lunar crust as the moon spun past. Rocks are solid, but not entirely inelastic. The tidal shift was small—nothing like the tides in our liquid oceans. Relaxation was slow—again, nothing like ocean tides. The tidal bulge swung ahead. The moon was skewed and Earth's gravity dragged on its hump. That put the brakes on and the moon's spin slowed. Eventually, it stopped running ahead of the pull of Earth, and the moon became tidally locked with its face toward us.

Tidal locking is common. It happens all over the solar system—and probably beyond. Mercury presents the same face to the sun all the time. Many moons face their planets. Tidal locking is a two-way street. Tiny Pluto and its moon are locked facing one another as they pirouette through space together. Larger planets take longer. Eventually Earth's rotation will match the moon's orbit. One side of the planet will have a moon, the other won't. Nothing to worry about, though. That will happen billions of years into the future—long after the apocalypse.

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