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{ Practical astronomy | Astronomy | The Moon
  | Physical ephemeris | Libration }


libration in longitude - Mare Crisium - and latitude - Aristoteles and Eudoxus
Libration of the Moon.

libration in longitude - naked-eye resolution
Libration of the Moon, to the naked eye.

On average, the Moon always shows the same side to Earth. Over millennia, the Earth's tidal forces have adjusted the lunar rotation period to equal its revolution period. That is to say, the Moon rotates in the same time as it revolves once around the Earth. It then shows the same side to the Earth at all times.

While this lock is precise on a monthly or long-term basis, there are deviations during parts of each month. This is called libration.

Galileo – one of the first astronomers to point a telescope at the Moon – denied the existence of libration in longitude, perhaps because he considered the lunar orbit circular. On the other hand, observers before Galileo might have noticed the libration even without a telescope. The second pair of images has been smoothed and re-sampled to a resolution similar to that of the naked eye.

Physical parameters:

Image parameters: