The lunar phases gradually change over the period of a synodic month (about 29.53 days), as the orbital positions of the Moon around Earth and of Earth around the Sun shift.
The Moon's rotation is tidally locked by Earth's gravity; therefore, most of the same lunar side always faces Earth. This near side is variously sunlit, depending on the position of the Moon in its orbit. Thus, the sunlit portion of this face can vary from 0% (at new moon) to 100% (at full moon). The lunar terminator is the boundary between the illuminated and darkened hemispheres.
Each of the four "intermediate" lunar phases is around 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 daylight, followed by 14.77 days of "night". The side of the Moon facing away from Earth is sometimes called the "dark side of the Moon", although that is a misnomer.
In western culture, the four principal phases of the Moon are new moon, first quarter, full moon, and third quarter (also known as last quarter). These are the instances when the Moon's ecliptic longitude and the Sun's ecliptic longitude differ by 0°, 90°, 180°, and 270°, respectively. Each of these phases occur at slightly different times when viewed from different points on Earth. During the intervals between principal phases, the Moon's apparent shape is either crescent or gibbous. These shapes, and the periods when the Moon shows them, are called the intermediate phases and last one-quarter of a synodic month, or 7.38 days, on average. However, their durations vary slightly because the Moon's orbit is rather elliptical, so the satellite's orbital speed is not constant. The descriptor waxing is used for an intermediate phase when the Moon's apparent shape is thickening, from new to full moon, and waning when the shape is thinning.
When the Moon as seen from Earth is a thin crescent, Earth as viewed from the Moon is almost fully lit by the Sun. Often, the dark side of the Moon is dimly illuminated by indirect sunlight reflected from Earth, but is bright enough to be easily visible from Earth. This phenomenon is called earthshine and sometimes picturesquely described as "the old moon in the new moon's arms" or "the new moon in the old moon's arms".
- The lunar phases gradually change over the period of a synodic month. Synodic refers to .
- Why does the same lunar side face the earth?
- The side of the Moon facing away from Earth is sometimes called the "dark side of the Moon", although that is a misnomer. What could misnomer mean in this statement?
- Why does the duration of each phase of the moon vary?
- What does the phrase " the old moon in the new moon's arms mean'?
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