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Bob Fulks wrote in response to my Julian Calendar post:
> I have been trying to find the accurate time between
> full moons and this post made me wonder if your expertise
> included this number...
That is a more complicated question than it might seem. Since the moon
shines by reflecting light from the sun (and a very small amount from the
earth), the phases are easily explained. However, the moon not only orbits
about the earth, but the earth rotates within on it's axis. It also orbits
about the sun. The vector sum of those motions causes points on the earth's
surface to move complexly with respect to distant stars (this is further
complicated by the tilt of earth's axis with respect to the plane of it's
orbit about the sun and other things). The moon's orbital period is
different depending on whether it is measured with respect to the earth's
center, a specified point on the surface of the earth, the sun, which the
moon also orbits, or distant stars.
The moon is called a new moon when it is positioned between the earth and
the sun, at which time the light of the sun does not reach the side of the
moon that faces the earth. At full moon the side of the moon facing the
earth is completely flooded with sunlight. This is the position where the
moon has half-completed a revolution about the earth. The interval between
new-moon and new-moon, or full-moon and full-moon, is called the synodic
month. It is 29 days, 12 hours, 44 minutes, and 2.8 seconds.
However, because the earth is spinning on its axis inside the moon's orbit
and also orbiting about the sun, an observer on the surface of the earth
will not be in the same position in space at the beginning and end of a moon
cycle about the earth, so the apparent period will depend on the motions of
both the observer and the moon and will be much different than the moon's
orbital period with respect to distant stars. The time required by the moon
to complete one revolution about the earth with respect to distant stars it
called a sidereal month. A sidereal month is 27 days, 7 hours, 43 minutes,
11.5 seconds.
These are not the only ways of measuring the moon's orbital period. There
is another moon period called an Anomalis month, and several others. I am
not an astronomer. I only have rudimentary knowledge of the fundamentals.
Maybe someone else on the list can provide better explanations.
-Bob Brickey
Scientific Approaches
sci@xxxxxxxxxx
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