Because of the phenomenon of libration, only 41% of the moon is actually always hidden from observers on Earth.

If you were to sit on the surface of the sun and look at the Earth through a telescope, you would observe it rotating both around the centre of the solar system and around its own axis. The side of the sphere facing you (the side experiencing daylight) would consist of a constantly shifting selection of Earth’s surface as it rotates: with new areas becoming lit in the west as sections in the east fall into shadow. You could watch as the eastern seaboard of North America came into illumination, then passed back into darkness as it spins away to the shadowed side of the planet once again.

By contrast, when you look up from Earth at the moon, the same face is basically presented all the time. Just as one side of the moon is always in view, there is a ‘dark side’ that is always hidden from the vantage point of an observer on Earth. This is because of a phenomenon called tidal locking. The moon rotates on its own axis at just the right rate so that, as it orbits the Earth, the same side is presented. There are, however, minor oscillations in this presentation. This is called libration, which derives from the Latin word meaning ‘to sway.’ You can see an animation of the phenomenon here: http://www.photoastronomique.net/geant/041200.html . It derives both from the fact that the moon’s axis is slightly inclined when compared to its orbit around the Earth and because the moon’s orbit around the Earth is slightly eccentric. Because of the cumulated rocking motion, it is actually possible to see 59% of the moon’s surface from the Earth.

This node has been modified from a post on my blog, at: http://www.sindark.com/2006/09/02/only-41-of-the-moon-is-dark/

Li*bra"tion (?), n. [L. libratio: cf. F. libration.]

1.

The act or state of librating.

Jer. Taylor.

2. Astron.

A real or apparent libratory motion, like that of a balance before coming to rest.

Libration of the moon, any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere.

 

© Webster 1913.

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