Lunar Phase Simulator – Background 4

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The earth's rotation explains the apparent motions of objects in the sky. As the observer rotates with the earth, objects appear to rise and fall below the horizon.

The rotation of the earth is also responsible for the apparent rising and setting of objects in the sky. Only half of space is visible to an observer on earth at any time — the other half is blocked out by the planet. The plane that separates the blocked and visible portions of space is called the horizon plane. As the observer rotates with the earth so does the observer's horizon plane, and as new objects are swept into view it appears to the observer that they rose from the horizon. Similarly, when objects that were visible cross the horizon plane they appear to set.

The interactive diagram below demonstrates the rising and setting of an object like the moon. From a suitable distance we can see that it is the observer who rotates, although to the observer it seems like everything else is rotating in the opposite direction.