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Lunar Libration

Bruce McClure
January 1, 2003

Libration is the slight rocking motion of the moon.

Hector Hugh Munro claimed, "A little inaccuracy sometimes saves a ton of explanation." No wonder it's said that the Moon keeps one face toward Earth, and that we see only 50% of the lunar surface. Actually, a slight rocking motion, called libration,* makes about 59% of the Moon's surface visible from our planet Earth -- though only 50% at any one time.

There are various causes for libration. There are also different kinds of libration: libration of longitude and libration of latitude.

Libration of longitude, the Moon's east-west wobble, is a product of the Moon's elliptical (elongated) orbit. Although the Moon's rotation is nearly constant, its orbital speed various, going fastest at perigee (Moon's closest approach to Earth) and slowest at apogee (Moon's farthest point from Earth).

At perigee or apogee, there is NO libration of longitude. Maximum librations are seen about one week AFTER perigee and one week AFTER apogee, each time revealing about 8 degrees of longitude on the Moon's far side. Following perigee, the Moon's rotation can't keep pace with its orbit, so a slice of the Moon's back side slips into view along the Moon's east (or right) limb; following apogee, the Moon's rotation surges ahead of its orbit, causing a sliver of the Moon's back side to appear along the west (or left) limb.

Libration of latitude, the north-south nodding, results primarily from the approximate 5 degree tilt of the Moon's orbital plane in respect to the ecliptic (Earth's orbital plane). Add to that, the approximate 1.5 degree tilt of the Moon's equator to the ecliptic, and you have the inclination of the Moon's equator to the plane of its orbit around Earth at some 6.5 degrees (5 + 1.5 = 6.5). Consequently, during the month, you can see a maximum of 6.5 degrees of latitude beyond the Moon's north pole, and a fortnight later, 6.5 degrees past the south pole.

Your position on Earth also has some, but significantly less, bearing on latitudinal libration. If you reside at far northern latitudes, you see further north on the Moon than someone in the Southern Hemisphere. Of course, the reverse is also true: someone in the Southern Hemisphere sees more of the Moon's southerly features.

Your positon also influences longitudinal libration -- though once again, rather marginally. At moonrise, you can make out a little more of the Moon's east (or top) limb; and at moonset, a little more of the Moon's west (and now at top) limb.

* libration comes from libra, the Latin word for scale or balance. According to Webster's dictionary, to librate means to move back and forth slowly like the beam of a balance in coming to rest.

copyright 2003 by Bruce McClure
Re-published on Astronomy Net with permission of the author.

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