From: Steve Beeson (beeson@asu.edu)
Date: Tue Dec 14 1999 - 07:47:28 PST
Date: Tue, 14 Dec 1999 08:47:28 -0700 From: Steve Beeson <beeson@asu.edu> Subject: RE: Full Moon Message-id: <8093ABAD9B81D211878200A0C9B406BAA7B75E@mainex3.asu.edu>
Geoff --
In order to really understand why the full moon will be so bright this year,
you need to understand two different kinds of motion. One, the motion of
the Earth/Moon system around the Sun; and two, the motion of the Moon around
the Earth. The first situation puts the Earth/Moon system closer to the sun
during the winter time (which is counterintuitive to people who think the
seasons are caused by the relative proximity of the Earth to the Sun, when
they are really caused by the 23.5 degree tilt of the N/S hemispheres away
or towards the Sun, which is a bigger effect, temperature-wise...). This
means the Moon reflects (in general) more sunlight during the winter.
Secondly, the Moon's orbit around the Earth isn't perfectly circular either.
Its ellipticity puts it closer at some points ('perigee') and farther at
others ('apogee'). On the 22nd, the Moon will be at perigee. (BTW, the
terms for when the Earth/Moon system is closer/farther from the Sun is
'perihelion' and 'aphelion'). This makes the Moon appear larger, by about
14% give or take.
So the combination of the two factors -- 1) Earth/Moon close to Sun, and 2)
Moon closest to Earth -- will make the full moon this month esp. bright and
large
Happy skywatching,
Steve Beeson
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