From:	EXOS%"green@noao.edu" 30-DEC-1988 13:20:24.87
To:	JMS
CC:	
Subj:	 Phases of the Moon

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Date:  Tue, 27 Dec 88 12:11:31 MST
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To:  jms@mis.arizona.edu

Hello Joel Snyder,
   Your questions about the moon's phases are reasonable and interesting.
That "high point" is called "transit" by astronomers, and is achieved by
every celestial object, including the moon.  It's a natural consequence of
their rising on the Eastern horizon and setting in the West; their highest
apparent elevation in the sky (off the horizon) occurs halfway in between.
When the moon is full, it is exactly on the other side of the Earth from the
Sun.  Since the Sun transits at about noon, the full moon transits at about
midnight.
   Interestingly enough, the elevation of the moon's transit varies a great
deal throughout the year, and through each month (or lunation).  The moon's
orbital plane is tipped by about 29 degrees with respect to the Earth's
equatorial plane.  Each month, the moon describes a complete circuit, appearing
at +29 degrees to -29 degrees with respect to the Earth's equator projected
onto the sky.  The Earth "carries" the Moon and its orbit around the Sun with
it, so once a year the full Moon transits near the "top" of its orbit at +29
degrees (almost overhead in Tucson).  Six months later, the Moon will transit
very low in the sky, at about -29 degrees (28 degrees above the horizon).
While the full Moon transits at midnight each month, then, the height of 
that transit varies from month to month.
   I'll be happy to provide an astronomy E-mail answering service.  Please 
give my regards to Bruce.
                                   Richard Green