2012 February 5
Lunation is the mean time of one lunar phase cycle, about 29.5 days. The moon's side that faces Earth never changes, but the manner in which sunlight is reflected off it differs on a night to night basis. Lunar libration is the cause of the rocking motion observed in the moon. There are three different types of libration: longitude, latitude, and Diurnal. The changes in longitude are caused by the moon's elliptical orbit, and the changes in latitude are due to the moon's inclined axis of rotation; however, diurnal libation is caused by the observer's position shifting as the Earth rotates. These three characteristic all contribute to the perceived rocking motion of the moon.
Click the image to see the rocking motion.
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