Why does a Full Moon appear at a position at 8:00 PM one night, and at the same position at 10:00 PM the next night?

Well...The Moon's sidereal month is 27 days long which determines its motion relative to the background stars. This means it moves 13 degrees per day ( actually 13.17), so that each day, it moves westward by this amount and on subsequent nights it will rise (13/360)x24hours = 0.87 hours later each night. So if you saw it at a specific sky position on one night at 8PM, you would normally see it at the same sky position at 8:50 the next night. Now, if on the second night you were in daylight savings time in the Fall, the actual clock time would be 7:50, or if you were setting your clocks forward in the spring, then on the second night it would be 9:50 for a difference of nearly 2 hours.


Copyright 1997 Dr. Sten Odenwald
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