SCIENCE OF THE SOLSTICE

As we've mentioned, the Romans celebrated a holiday know as the
Saturnalia beginning on the Winter Solstice. The word Solstice literally
means "Sun, standing-still" In order to work some history into this
science article, I should point out that years ago, I was the Product
Marketing Manager for the Solstice enterprise management product line at
Sun Microsystems. Now on to the science.

Earth enjoys different seasons because the planet is tilted 23 degrees
and 27 minutes off the perpendicular to the plane of orbit.  This means
that the earth revolves like a tilted spinning top.  The Winter Solstice
is the shortest day of sunlight as the Sun is at its lowest arc in the
sky. The farther north one is from the Equator, the more pronounced this
is in Winter.

This year will be the first full moon to occur on the Winter Solstice,
December 22nd (7:44 am Universal Time), commonly called the first day of
Winter. Since a full moon on the Winter Solstice occurs in conjunction
with a lunar perigee (the point in the moon's orbit that is closest to
Earth), the moon will appear about 14% larger than it does at apogee
(the point in its elliptical orbit that is farthest from the Earth).
Since the Earth is also about three million miles closer to the sun at
this time of the year than in the Summer, sunlight striking the moon is
about 7% stronger making it brighter. (Note: Summer in the Northern
Hemisphere is hotter because of the more directly perpendicular angle 
that the sunlight hits the Earth, not the closeness to the Sun. Summer
in the Southern Hemisphere is both closer to the Sun and has the
direct perpendicular angle of sunlight.)

Also, this will be the closest perigee of the Moon of the year since the
moon's orbit is constantly deforming. If the weather is clear and there
is a snow cover where you live, as I do in Colorado, it is believed that
even car headlights will be superfluous. (And in Colorado at that time of
the year, some people don't use headlights anyway :-)

In simple terms it will be a very bright full moon, much more than the
usual AND it hasn't happened this way for 133 years! Our ancestors 133
years ago saw this. Our descendants 100 or so years from now will see
this again. History, science, and prediction of the future, all in the
same article!

FACT: On December 21, 1866 the Dakota Sioux took advantage of this
combination of occurrences and staged a devastating retaliatory ambush
on soldiers in the Wyoming Territory. 

     Bill Petro, your friendly neighborhood historian
     www.billpetro.com/holidayhistory