MadSci Network: Astronomy

Re: Moonrising and moonsetting times

Date: Thu Jul 30 18:59:34 1998
Posted By: Brian Kane, Post-doc/Fellow, Astronomy, AstroPlace, Inc.
Area of science: Astronomy
ID: 901241950.As

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

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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