Physics Illustrated

WHERE IS THE CELESTIAL EQUATOR?

The celestial equator is an imaginary plane that extends infinitely into space from every point on the Earth's equator. This allows astronomers to separate the southern sky from the northern sky and to locate and plot points in space.

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WHY DOES THE MOON CHANGE SHAPE?

It doesn't. What we're actually seeing is varying amounts of sunlight reflected off the Moon as it orbits the Earth every 29� days. When the Moon is between the Earth and the Sun, its sunlit side faces away from us and we see a dim Moon illuminated only by light reflected from the Earth. On each succeeding day, as the Moon makes its way around the Earth, a larger sunlit portion is visible to us. This portion gets progressively larger until about two weeks later, when the Earth is between the Sun and the Moon and the entire lit side is visible.

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