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Nearside


The near side of the Moon is the lunar hemisphere that is permanently turned towards the Earth, whereas the opposite side is the far side of the Moon. Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a situation known as synchronous rotation or tidal locking. The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions cause the lunar phases. The unilluminated portions of the Moon can sometimes be dimly seen as a result of earthshine, which is sunlight reflected off the surface of the Earth and onto the Moon. Since the Moon's orbit is both somewhat elliptical, and inclined to its equatorial plane, libration allows up to 59% of the Moon's surface to be viewed from Earth (but only half at any instant from any point).

The near side of the Moon is characterized by large dark areas that were believed to be seas by the astronomers who first mapped them, in the 17th century (notably, Giovanni Battista Riccioli and Francesco Maria Grimaldi). Although it was found out later that the Moon has no seas, the term "mare" (plural: maria) is still used. The lighter toned regions are referred to as "terrae", or more commonly, the "highlands".

The image of the Moon here is drawn as is normally shown on maps, that is with north on top and west to the left. Astronomers usually turn the map over to have south on top, as to correspond with the view in most telescopes which also show the image upside down.

West and east on the Moon are where you would expect them, when standing on the Moon. But when we, on Earth, see the Moon in the sky, then the east–west direction is just reversed. When specifying coordinates on the Moon it should therefore always be mentioned whether geographic (or rather selenographic) coordinates are used or astronomical coordinates.


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Wikipedia

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