Re: Eclipses and the orbital plane of the Moon

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 17 Aug 1999, Lois Breur Krause wrote:

> the orbit of the moon is in a plane a bit off from that of the earth's
> orbit of the sun, as you have suggested.  find a good map of the solar
> system and you'll see why we don't get eclipses every month.

Such a consideration will show why eclipses only occur approximately every
six months.  But it will further beg the question of why eclipses don't
only occur *exactly* every six months.  For instance, we just had an
eclipse so why isn't it common knowledge that eclipses only occur in
August and February?  The answer is that the Moon's orbital plane
maintains a constant 5.2 degree (or about ten "moon diameter") tilt to the
ecliptic but also precesses relative to the ecliptic taking 18.6 years to
make one full cycle and causing the two eclipse seasons to occur 19.6 days
earlier each year.

I've always considered this rapid precession of the Moon's orbit to be a
stunning reminder of the relative strength of the Sun's gravitational
effect on the Earth-Moon system somewhat akin to the fact and the reason
for the fact that the Moon's orbit is always concave toward the Sun.

Here's another Moon question to think about:  We all know that the path of
the Sun is high in the sky in the summer and low in the sky in the winter.
What about the path of the full Moon in summer and winter?  (And how many
people ever notice this?)

John
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