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A Straight Dope Classic from Cecil's storehouse of human knowledge
10-Apr-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? --Trigby P.,
Northridge, California
Cecil replies:
You're right to be suspicious about this, spud, but let's clear up one thing first: there
is 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 just
four inches per month, 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.
--CECIL ADAMS
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