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{ Practical astronomy | Astronomy | The Moon | 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. There are two imperfections in this synchronisation of spin and orbit. First, the rotation is at constant speed while the orbital motion speeds up near perigee and slows down near apogee. Second, the spin axis is not exactly perpendicular to the orbital plane.

The first imperfection leads to libration in longitude, allowing us to see a bit more on the left or on the right in different parts of the month. This can be seen best in the shape of Mare Crisium. In the right image it is closer to the limb and looks less circular.

The second imperfection leads to libration in latitude, allowing us to see a bit more at the top or at the bottom. In the image pair this is quite apparent from the two craters toward the North. The larger one is called Aristoteles, the smaller Eudoxus.

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 Galilei 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.

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