A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why does the same side of the moon always face the earth?

April 10, 1987

Dear Cecil:

Why does the same side of the moon always faces the earth? It seems like quite a coincidence that it should rotate so perfectly in sync with us. Is there any slippage, so that parts of the dark side of the moon are slowly being revealed to us?

Cecil replies:

You're right to be suspicious about this, spud, but let's clear up one thing first: there's no "dark side of the moon," the popular expression notwithstanding. All of the moon is illuminated at some point during the month-long lunar day. It's just that we can't see when it's high noon on the back side.

As for the moon's rotation, you're right in thinking the timing is a little too neat to be coincidental. It was different once upon a time. Billions of years ago experts think the moon was much closer to the earth than it is now and rotated much faster, so that over time the entire lunar surface could be seen from earth.

But "tidal friction" slowed the moon down. The earth's gravity caused the side of the moon closest to us to bulge outward, just as the moon's gravity causes our oceans to bulge and create tides. The continual deformation of the lunar crust as it rotated relative to the earth acted as an interplanetary brake, and eventually the moon slowed so that the same side always faced toward the earth. (The moon also got farther away.) The result is called "captured" or "synchronous" rotation, and it's common throughout the solar system.

Eccentricities in the lunar orbit and whatnot periodically do bring some of the moon's backside into view, a process called "libration." In all about 59 percent of the moon's surface is visible at some point. But you'd better look while there's still time — the moon continues to recede from us. Admittedly this occurs at the rate of less than four centimeters per year, so it's not as though you don't have time to finish reading the newspaper. The pull of the moon has also had a braking effect on the earth, causing our rotation to slow (and thus our day to lengthen) at the rate of one second per hundred thousand years. Good news, at least in the long term, for those who complain there's never enough time in the day.

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