The
lunar phase or
phase of the moon is the shape of the illuminated (sunlit) portion of the
Moon as seen by an observer on
Earth. The lunar phases change cyclically as the Moon
orbits the Earth, according to the changing positions of the
Moon and
Sun relative to the
Earth. The Moon's rotation is
tidally locked by the Earth's gravity, therefore the same lunar surface always faces Earth. This face is variously sunlit depending on the position of the Moon in its orbit. Therefore, the portion of this hemisphere that is visible to an observer on Earth can vary from about 100% (
full moon) to 0% (
new moon). The
lunar terminator is the boundary between the illuminated and darkened hemispheres. Each of the four "intermediate" lunar phases (see below) is roughly seven days (~7.4 days) but this varies slightly due to the elliptical shape of the Moon's orbit. Aside from some craters near the lunar poles such as
Shoemaker, all parts of the Moon see around 14.77 days of sunlight, followed by 14.77 days of "night". (The side of the Moon facing away from the Earth is sometimes called the "dark side", which is a misnomer. It receives just as much sunlight as the Earth-facing side - actually slightly more, since the Earth never obscures the Sun from it.)