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The phases of the moon refer to the moon's changing appearance during the course of a lunar month. A lunar month is the time taken by the moon to complete one revolution around the earth. A lunar month is of 27.3 days, but the earth's simultaneous motion around the sun makes it seem as if the moon takes two days longer, around 29.5 days.
The moon gets its illumination from the sun. The side facing the sun is the one lit up and the side facing away from the sun is in darkness. Only the bright portion of the moon is visible from earth. During a lunar cycle, as the moon revolves around the earth, the bright portion of the moon is seen at different angles from the earth. This makes it appear like the moon is changing its shape.
Eight phases of the moon occur in a lunar cycle. They are New Moon, Crescent Moon, First Quarter Moon, Waxing Gibbous Moon, Full Moon, Waning Gibbous Moon, Last Quarter Moon, Crescent Moon and New Moon. The phases of the moon keep repeating in an endless cycle, but, since this cycle takes 29.5 days, the moon phases do not happen on the same days every month.
The New Moon is the first of the eight phases of the moon. It is the result of the moon coming between the earth and the sun. The side of the moon lit by the sun is away from the earth, the dark portion faces the earth and so there appears to be no moon in the sky. The moon then changes position and a sliver of it lit by the sun becomes visible from earth. This is the Crescent Moon.
When half of the moon is visible it is the First Quarter Moon. At this time the moon has completed one quarter of its revolution around the earth. When more than half of the moon becomes visible, it is the Waxing Gibbous moon.
The Waxing Gibbous waxes or grows and finally the entire sunlit disc of the Full Moon makes its appearance. It is the only phase where the moon is visible in the sky throughout the night. With the Full Moon, the moon completes one half of its revolution around the earth.
As the moon begins the next half of its revolution, the portion of moon that is visible from earth begins to grow thinner. The Full Moon wanes and the sliver of darkness of the Waning Gibbous Moon appears. When half of the moon turns dark, it is the Last Quarter Moon. This transforms into the bright sliver of the Crescent Moon and finally to total darkness with the New Moon.