The Earth turns once on its axis every 24 hours. Since one revolution equals 360 degrees, the Earth rotates 360 / 24 = 15 degrees per hour. In the horizon system, a star on the celestial equator would be moving to the west at 15 degrees per hour.

The Moon, as we all know, orbits the Earth. It takes 27.32 days to return to the same right ascension. So that means that it moves 360 degrees / (27.32 days * 24 hours / day) = 0.549 deg/hr to the east against the stars, on average. If the Moon is moving 15 deg/hr to the west owing to the rotation of the Earth, but 0.5 deg/hr to the east owing to its orbit around the Earth, then its net motion is 14.5 deg/hr to the west. This is why, on average, the Moon sets about 12 hours and 26 minutes after it sets. (If the Sun is on the celestial equator -- the first day of spring or the first day of autumn -- it sets 12 hours after it rises.)

In the graph below we show the Moon's azimuth and elevation angle above the horizon on June 17, 2008 UT, as viewed from Socorro, New Mexico. We use the RA-DEC of the Moon at 9 PM as a reference. As the night progresses, the stars and the Moon are moving to the west with respect to the horizon. But the Moon is found further and further east compared to the RA-DEC it had at 9 PM.

Go back to Kevin Krisciunas home page by clicking here.