When the moon hits your eye, it's always the same old scene

By Ben Harder
Tuesday, July 6, 2010

If you moved to the moon, you'd have to choose between two basic types of real estate. To property owners living in one hemisphere, Earth would at all moments be visible overhead, forever suspended in the sky. From properties in the opposite hemisphere, Earth would be perpetually hidden below the horizon, never to be seen.

Lunar Realtors would, of course, call the former "Earth-view properties." The latter they might advertise as having "No planetary neighbors in sight!"

These stark zoning difference between the moon's hemispheres may seem pretty strange to earthlings. We're used to witnessing both the sun and the moon rise and set daily. But here's a subtler weirdness you might have never thought about: Although the moon cycles through phases -- going from new to full and back again -- it always puts the same side, the same face, toward us. The "man in the moon" may be easiest to see when the moon is full, but his shadowy visage always peers at us, even when only a sliver of it is illuminated.

The explanation for both his steadfast vigil and the moon's odd real estate market is that the time the moon takes to spin once around its axis and the time it takes to complete each revolution around Earth are exactly the same: 27.3 Earth days. As a result, the man-in-the-moon side of the satellite always faces earthward. The other hemisphere always faces away.

Celestial coincidence? No. If anything, it's the work of fate.

In the argot of planetary scientists, the moon is "tidally locked" to Earth. Tidal locking is thought to be common throughout the galaxy, and it can affect both how a moon orbits its planet and how a planet orbits its star. Over eons, the larger body's gravitational tug exerts a slight but steady braking effect on whatever spin the smaller partner starts with, until eventually the smaller partner's rotation falls into synchrony with the larger.

The physics behind tidal locking are a bit complicated, but the principles are fairly straightforward: They begin with the tide and end with the locking.

You see, tides are pretty much a fact of life for planets and moons, even ones that don't have oceans. On Earth, the oceans' ebb and flow are the most obvious signs of the tidal forces produced by the moon's gravity. But rock and magma also respond to the tides, and as a result Earth is forever bent out of shape. Rather than being a perfect sphere, it's slightly squished. Tide-induced bulges form along an axis that points toward the moon, and these bulges circle around Earth each day as the rotation of the planet changes its orientation to the moon.

Once upon a time, before our rocky, roundish moon became tidally locked, tidal bulges also swept continually across its surface, tracking the shifting position and pull of Earth. The bulges, however, lagged slightly behind the gravitational forces producing them, because rock is slow to stretch and compress.

Confused yet? Good, because this next part is sure to be mind-warping: Because of the misalignment between the axis of the moon's bulge and the axis of the gravitational pull exerted by Earth, the moon's rotation gradually slowed down. Torque, they call it.

Think about it like this: If, when walking your dog, you pull persistently on the leash attached to its neck, the dog eventually will end up facing you. In the same way, Earth's relentless pull (the leash) eventually forced the moon to point its bulge directly at us (because the bulge is where Earth's pull is strongest). The dog can keep trying to twist away from you, just as the moon kept spinning, but eventually Earth's gravity pulled the bulge to a halt and the satellite became tidally locked.

There's nothing unique about how our planet subdued its moon's spin. Both of Mars's moons, Phobos and Deimos, are tidally locked to the Red Planet; they orbit with one side always facing toward the planet and the other always facing away. And numerous satellites of Jupiter, Saturn and the other gaseous planets are locked as well.

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