99.5% Waning Gibbous, 03/29/2013, 6:00 to 6:15 AM
A
lunar phase or
phase of the moon is the appearance of the illuminated (sunlit) portion of the
Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon
orbits the Earth, according to the changing relative positions of the
Earth,
Moon, and
Sun. The half of the lunar surface facing the Sun is always sunlit, but the portion of this illuminated 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 unilluminated hemispheres. 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" (there is no permanently "dark side" of the Moon).