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Lunar Libration: Earth Image of the Week February 10, 2012
Satellite Image
The moon passing through one full "lunation."
Some of the youngest schoolchildren learn that the moon keeps one side pointing toward the Earth at all times. But time-lapse photography reveals that it’s really not quite that simple.

The moon generally has one hemisphere facing our planet due to a gravitational influence known as tidal locking.

But subtle orbital features actually allow about 59 percent of it to be seen by discerning observers on Earth.

Such is the case in the full lunar cycle, or lunation, revealed in the series of images to the right.

Lunar libration causes an apparent slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.

The moon's libration actually involves three different orbital aspects.

• Libration in longitude results from variations in the moon's orbit around Earth that sometimes cause it to lead or lag in its orbital position.

• Libration in latitude results from a slight difference in geometry between the moon's axis of rotation and its orbit around Earth. This is somewhat similar to how the seasons result from Earth's tilt as it orbits about the Sun.

• Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining the centers of the Earth and moon. This allows the observer to first look around one side of the Moon and then around the other, because the observer is on the surface of the Earth and not at its center.

Full story and image: NASA