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Path: utzoo!watmath!clyde!floyd!harpo!seismo!flinn
From: flinn@seismo.UUCP (E. A. Flinn)
Newsgroups: net.micro
Subject: Re: astronomical references/algorithms wanted
Message-ID: <717@seismo.UUCP>
Date: Mon, 19-Mar-84 09:55:13 EST
Article-I.D.: seismo.717
Posted: Mon Mar 19 09:55:13 1984
Date-Received: Tue, 20-Mar-84 01:22:41 EST
References: <17663@sri-arpa.UUCP>
Organization: Center for Seismic Studies, Arlington, VA
Lines: 21

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On computing the phase of the moon:  it was said -

>>I computed the phase of the moon by a fairly simple algorithm:
>>A reference handbook lists the lunar month at 2551442.8 seconds.
>>Find the number of seconds since an arbitrary time in the past
>>(such as midnight Jan 1, 1980).
>>Take that time modulo the length of a lunar month.  You now have
>>the number of seconds into this month's cycle.  You can then
>>check for which quarter you are in.

You should be aware that the Moon is perturbed in its orbit by the
gravitational attraction of other bodies in the solar system, and by
the deviations from spherical symmetry of the Sun, Earth, and other
planets.  The length of the synodic month varies a good deal (of
the order of 1% or so), and if you use your algorithm over an interval
of several years you'll begin to get the phases wrong.

		-- Ted Flinn