Phase of the Moon

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 of the Moon", although that is a misnomer.)

In Western culture, the four principal lunar phases are new moon, first quarter, full moon, and third quarter (also known as last quarter). These are the instances when the Moon's apparent geocentric celestial longitude minus the Sun's apparent geocentric celestial longitude is 0°, 90°, 180° and 270°, respectively. Each of these phases occur at slightly different times when viewed from different points on the Earth. During the intervals between principal phases, the Moon appears either crescent-shaped or . These shapes, and the periods of time when the Moon shows them, are called the intermediate phases. They last, on average, one-quarter of a synodic month, roughly 7.38 days, but their durations vary slightly because the Moon's orbit is slightly elliptical, and thus its speed in orbit is not constant. The descriptor waxing is used for an intermediate phase when the Moon's apparent size is increasing, from new moon toward full moon, and waning when the size is decreasing.

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