What causes the phases of the Moon? You have probably noticed that the Moon does not rise and set at the same times or sky locations from day to day. You are seeing the effect of the Moon revolving around the Earth. Since almost all of the Moon's light is reflected sunlight, the portion of the Moon which is not facing the Sun at any given time will be almost completely dark. Contrary to the assumptions of some, the side of the Moon which faces away from Earth is not perpetually dark. When the Moon is between the Earth and the Sun (New Moon), the "far side" is completely sunlit.

We always see the same side of the Moon, but that does not mean that the Moon does not rotate. If the Moon did not rotate, we would be able to see the whole surface of the Moon within one revolution. Actually, the Moon's rotation period corresponds almost exactly to its period of revolution around Earth, so that the same half of the Moon is always facing us. Small changes in the attitude of the Moon, known as librations, allow us to see slightly more than half of the Moon in the long run.

As the Moon revolves around the Earth, its angle with respect to the Sun changes. When the Moon is very near the Sun, only a small portion of the near side is lit; we see a thin crescent. The round edge of the crescent points down to the horizon, revealing where the Sun has set or is due to rise. After the New Moon, we first catch a glimpse of the thin crescent in the evening, growing fatter and appearing higher in the sky as the days pass. When the Sun-Earth-Moon angle is 90 degrees, half of the near side of the Moon is lit. This situation occurs a quarter of the way through the Moon's "monthly" cycle, and again three quarters of the way through, so these phases are known as First Quarter and Last Quarter. The days following first quarter are marked by gibbous phases, as the near side of the Moon progresses towards fullness. Full Moon, when the entire near side of the Moon is lit as seen from Earth, comes at the midpoint of the period of revolution. After Full Moon, the Moon wanes, the light attenuated as more of the near side turns away from the Sun. The waning phases progress in exactly the opposite fashion of the waxing phases; i.e., from gibbous to half-lit (Last Quarter) to crescent to New. The Moon rises later each night, so that after Last Quarter it will only be visible in the morning sky.

I mentioned that *almost* all of the Moon's light comes from the Sun. A small amount also comes from sunlight reflected off the Earth. When the Moon is a thin crescent, the dark portion that faces Earth is faintly visible as a result. This phenomenon is known as Earthshine, or "the old Moon in the new Moon's arms".

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