UpDate - Vol. 11, No. 14, Page 6
December 12, 1991
Moon watch; Minor eclipse to occur just before winter solstice

     December is the month when the Earth's northern hemisphere is
faced away from the sun more than at any other time in our annual
circuit of that quintessential source of our light and heat. What
a difference that 23-degree tipping of the Earth's axis makes to
our lives! Since the autumnal equinox of Sept. 23, we have been
effectively transported 23 degrees farther north, equivalent to a
distance of 1,600 miles. Of course, the people 1,600 miles north of
us have been similarly transported to even colder climates.
     Whereas on Sept. 23, the sun at midday was 50 degrees above
the southern horizon (more than half-way to the zenith), now the
highest the sun stands is 23 degrees less, or only 27 degrees up
from the horizon. On the autumn equinox, the sun rose directly in
the east and set due west, allowing it to be in the sky for half of
the 24-hour day (equinox means an equal interval of nighttime to
daytime). Now we find the sun rising notably south of east,
crossing the sky in a much shorter and lower arc to set south of
west after being visible for only 9 1/2 hours. The date when the
sun is lowest is the date of winter solstice-this year on Dec. 22.
     All this is pretty common, an annual event well known to
people since prehistoric times (as we infer from such things as
their layout of stone circles to mark the significant rising and
setting points on the horizon). But, this year, the event is
enhanced by a coincidental eclipse of the moon.
     The eclipse is a minor one with only a tiny part of the moon
entering the full shadow of the Earth. Moreover, since this occurs
between 5 and 6 in the morning, with the maximum of 9 per cent of
the moon's diameter covered at 5:33 EST, few people will find it
worth the trouble to observe.
     However, what intrigues me is the DATE on which this lunar
eclipse occurs: the morning of Saturday, Dec. 21, when the sun is
less than a day away from the point of winter solstice. That means
that the moon at that time is almost exactly at the point in the
sky that we call summer solstice!
     Visualize the situation: The sun, Earth and moon in so nearly
a straight line that the moon touches the long shadow of the Earth.
In such a circumstance, the moon must appear as full. In fact, it
can scarcely be fuller than when we are so directly between it and
the sun, the sun's illumination coming from directly over our
shoulder, so to speak.
     Moon and sun are then 180 degrees apart in the sky, directly
opposite one another. That puts the moon where the sun will be half
a year later, on June 21, the date of summer solstice. On that date
the sun will rise north of the east point on the horizon, will
cross the sky in a long, high arc reaching a noontime altitude of
73 degrees (50 PLUS 23), and set north of west for a daylight
interval of 15 hours.
     Having a full moon in this position is not only unusual but
leads to several occurrences being near their maximum possible.
Moonrise occurs about as far north of east on the horizon as it
can. Watch where it comes up on the evenings of Dec. 20 and 21;
this is where the sun will rise on June 21.
     By being in the sun's June 21 position, the full moon this
month will reach unusual heights in the sky. Seldom do we see a
full moon so nearly overhead as it will be at midnight on Dec.
20-21.
     Furthermore, since the moon will be mimicking the sun's June
21 crossing of the sky, our mid-winter celebration this year will
be illuminated by a full moon that is in the sky for 15.5 hours,
nearly the maximum continuous interval possible from our latitude.
     Our map shows the moon's position against the background of
stars at the time of greatest eclipse, 5:33 a.m., Dec. 21. It is
just entering the constellation of Gemini, almost touching the toe
of the stick figure of Castor. This is the location of the summer
solstice, where the sun will be on June 21, but which the daytime
brightness then prevents our seeing. Now we can see where it lies
in relation to the constellations of Orion, Taurus and Gemini, but
even the moon has sufficient brightness to make the fainter stars
difficult to discern.
     Because of the moon's motion around the Earth, its exact
location amidst the stars depends on when you look. From Dec. 20 to
Dec. 22, it moves from Taurus to the middle of Gemini. Gemini is
Latin for twins and is the ancient name given to this region of the
sky in honor of the twin gods, Castor and Pollux, of the Romans.
The two bright stars of Gemini are appropriately named Castor and
Pollux. Although there is no reason to expect the star group to
look like its honorific name, we can sometimes connect the stars
mentally into memory aids like the stick figures of the map. The
stars Castor and Pollux mark the heads of the respective figures of
the twins.
     These constellations seem almost to be using the moon as a
ball for some celestial game. Taurus, the bull, has butted the moon
toward the twins. Orion reaches up but it is too high to catch. On
Dec. 21 Castor misses kicking it and the moon-ball gets by Pollux
on Dec. 22.
     A week earlier, when the moon is only first quarter and not
yet to this part of the sky, there may be a fine meteor shower that
seems to come from the direction of Gemini. This annual shower is
called the Geminids and should be best seen from midnight to dawn
on the morning of Dec. 14.
                                        - Richard B. Herr,
                                        Associate Professor of
                                        Physics and Astronomy