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Planetary Science
The Moon

Why do the sun and moon seem to go across the sky?


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December 20, 2009 11:09PM

In both cases it is called 'apparent' motion. The movement that appears to be happening is not truly happening. The observed movement is due to the rotation of the earth on its axis. It is easy to think of the sun being motionless relative to the rest of the solar system. The interesting thing is that the apparent motion of the moon is the opposite of its true motion. The moon is moving from west to east around the earth, but because the earth's rotation is relatively faster, the moon appears to be moving from east to west, like the sun.
The Sun and Moon appear to move across the sky because the Earth is spinning at a rate of 15 degrees per hour. When viewed from a merry-go-round, the buildings appear to move; but it is actually you and the merry-go-round that is moving. The buildings are standing still.

The Moon does move, but at a speed of about 12 degrees per DAY, much more slowly than the Earth spins.