|MadSci Network: Astronomy|
The Moon appears to rise and set because the Earth is rotating on its axis. But the Moon is also revolving about the Earth, making one revolution in a period equal to about one month. (In fact, the words "month" and "moon" are linguistically related.) The Moon revolves in its orbit in the same direction in which the Earth rotates on its axis. Because of this the Moon appears to rise slightly later---about one hour and fifteen minutes later---each night. On days when the moon is scheduled to rise later than 10:45 PM local time (in order words, less than one hour and fifteen minutes before midnight), the next rising will occur after midnight of the following day, so that the intervening day has no moonrise. Other factors can influence the timing of moonrise and moonset, including the inclination of the Moon's orbit relative to the plane made by the Earth's orbit around the Sun. The Moon orbits with a tilt of 5 degrees relative to the Earth-Sun plane, so at times it is said to "ride low," and at times to "ride high." When it rides low, it may appear to rise slightly later due to its rising at a shallow angle and there being obstructions like trees and buildings near the horizon. Furthermore, moonrise/set time is determined by where you are in your local time zone. People on the eastern edges of each time zone see the moon rise earlier than those on the western edges. For instance, the Moon may be scheduled to rise on a given night at 10:30 pm EDT. Observers in Down East Maine would actually see it rise at almost 10:00 pm, while those in Ohio would see it rise after 11 pm. So Maine observers would see the Moon rise the following evening, but Ohio observers would not see the Moon rise the next evening. Likewise, the further north you are above the Equator, the more shallow will be the Moon's path as it rises, giving rise to a situation similar to that caused by the tilt of the Moon's orbital axis.
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