One of the questions I am most frequently asked by the public is, "Why does the moon have different shapes?" If one has never studied the moon in any detail, this is an understandable question. And with the help of a special diagram I hope to shed some light on the subject.

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It's important to understand that while the Earth is rotating counterclockwise (eastward) on its axis, the Moon is revolving counterclockwise around the Earth. And because the Moon is gravitationally locked to the earth, we always see the same side or half of the moon from our earthly perspective. The lunar halves that are in sunlight and dark are the hemispheres that face toward and away from the Sun respectfully, and are always changing as the Moon moves around the Earth. Contrary to popular belief, there really is no dark side of the Moon or side that is always dark. Since the Moon is gravitationally locked to the earth, as it revolves around the earth, all of its faces receive sunlight during a portion of its 28-day cycle.

Use the illustration above to visualize the lighting of the Moon's faces as it revolves around the Earth. A New Moon occurs when the near side (the side we see) faces away from the Sun. A few days later the Moon is a Waxing Crescent, where the bright area, seen from our perspective, is getting larger. As the Moon continues to move around its orbit and the Earth-Moon line is at right angles to the Earth-Sun line, we see a Half Moon, which we call the First Quarter Moon. As it continues around the Earth and the bright part of the Moon is larger than a quarter, but less than full, it is called a waxing Gibbous Moon. When the Moon is opposite the Sun, the lunar hemisphere that faces Earth is fully lit and is known as a Full Moon. As the Moon continues further around its orbit, it becomes a Waning Gibbous, Last Quarter, and then Waning Crescent. (Waning means getting smaller.) It then becomes a New Moon and the cycle of phases starts over again.