Concept
The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a situation known as tidal locking. The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions cause the lunar phases. Sometimes the dark portion of the Moon is faintly visible due to earthshine, which is indirect sunlight reflected from the surface of 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 (though only half at any moment from any point). 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 traditionally turn the map to have south on top to correspond with the northern-hemisphere view in astronomical telescopes, which typically show the image upside down. West and east on the Moon are where they would be expected, when standing on the Moon. But when the Moon is seen from Earth, then the east–west direction is reversed. When specifying coordinates on the Moon it should therefore always be mentioned whether geographic (or rather selenographic) coordinates are used or astronomical coordinates. The Moon's actual orientation in Earth's sky or on the horizon depends on the viewers geographic latitude on Earth. In the following description a few typical cases will be considered. On the north pole, if the Moon is visible, it stands low above the horizon with its north pole up. In mid northern latitudes (North America, Europe, Asia) the Moon rises in the east with its northeastern limb up (Mare Crisium), it reaches its highest point in the south with its north on top, and sets in the west with its northwestern limb (Mare Imbrium) on top. On the equator, when the Moon rises in the east, its N — S axis appears horizontal and Mare Foecunditatis is on top.
