Near side of the Moon
The near side of the Moon is the hemisphere of the Moon that is facing Earth. While Earth keeps turning through its near side to the Moon, changing in the course of a day the part it faces toward the Moon, the Moon keeps the same surface oriented to Earth. This is due to the Moon rotating on its axis at the same rate that the Moon orbits the Earth—a phenomenon known as tidal locking. The opposite hemisphere is the far side.
The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions from Earth cause the lunar phases. The near side when dark 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.
Orientation
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 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 the Moon rises in the east with its northeastern limb up, it reaches its highest point in the south with its north on top, and sets in the west with its northwestern limb on top.
- On the equator, when the Moon rises in the east, its N — S axis appears horizontal and Mare Foecunditatis is on top. When it sets in the west, about 12.5 hours later, the axis is still horizontal, and Oceanus Procellarum is the last area to dip below the horizon. In between these events, the Moon reached its highest point in the zenith and then its selenographic directions are lined up with those on Earth.
- In mid southern latitudes the Moon rises in the east with its southeastern limb up, it reaches its highest point in the north with its south on top, and sets in the west with its southwestern limb on top.
- On the south pole the Moon behaves as on the north pole, but there it appears with its south pole up.