In addition to these actual changes in velocity, there is an apparent change caused simply by its being nearer or further when it is closer any motion that it has looks faster in angular terms than when it is further away. Similarly, during the half orbit that it is receding from us, our mutual gravitational pull decelerates the Moon, causing it to move slower and slower, until at the furthest point in the orbit, or apogee, it is moving about 6% slower than its average motion. During the half orbit that it is approaching us, our mutual gravitational pull accelerates the Moon, causing it to move faster and faster, until at the closest point in the orbit, or perigee, it is moving about 6% faster than its average motion. During half its orbit it is approaching us, and during the other half it is moving away from us. As a result, during each orbit the Moon's distance varies by twice that 12000 miles. First, the orbit of the Moon is an ellipse and is not centered on the center of the Earth, but on a point about 12000 miles from the center of the Earth. The Moon's Variation in Distance and SpeedĪlthough the Moon has an average motion of 13.2 degrees per day, this motion varies for two reasons. As a result, it rises (and sets) later and later every day, until after about 27 days, when it has gone once around the sky relative to the stars, it is back in its original position, rising and setting at its original time(s). Since its eastward motion averages 13.2 degrees per day and the Earth takes 4 minutes to rotate through one degree, it takes about 53 minutes (13.2 times 4) for the Earth to rotate through this extra angle which means that on the average the Moon crosses the sky once every 24 hours and 49 minutes (53 minutes longer than the stellar "day").
The stars go once around the sky in 23 hours 56 minutes (approximately), so the Moon, moving more slowly to the west, takes longer than this. This means that it still rises in the east and sets in the west like the stars, but a little later each day So though moving to the east from day to day, it still has a net motion toward the west each day. The Moon's eastward motion is much slower than the sky's westward motion.
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