Scouts-L Mail Archive for December of 1999: Full Moon Brightness (Long)
Full Moon Brightness (Long)
Ronald W. Fox
Tue, 21 Dec 1999 16:00:26 -0600
Here's a perspective on the upcoming full moon....
SKY & TELESCOPE'S NEWS BULLETIN - DECEMBER 17, 1999
For images and Web links for these items, visit http://www.skypub.com
Vote for the hottest new astronomical products of 1999 from those
selected by the editors of Sky & Telescope -- or suggest one of your
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BRIGHTEST MOON IN 133 YEARS?
Suddenly a lot of people are asking this question: Will the full Moon
of December 22nd be the brightest full Moon in 133 years? They're
asking, apparently, because of an article in the Old Farmer's Almanac
that is being widely circulated by e-mail.
According to Roger W. Sinnott, associate editor of Sky & Telescope
magazine, the answer is unequivocal: No!
It is true that there is a most unusual coincidence of events this
year. As S&T contributing editor Fred Schaaf points out in the
December 1999 issue of Sky & Telescope, "The Moon reaches its very
closest point all year on the morning of December 22nd. That's only a
few hours after the December solstice and a few hours before full
Moon. Ocean tides will be exceptionally high and low that day."
But to have these three events -- lunar perigee, solstice, and full
Moon -- occur on nearly the same day is not especially rare. The
situation was rather similar in December 1991 and December 1980, as
the following dates and Universal Times show:
Event Dec. 1999 Dec. 1991 Dec. 1980
Full Moon 22, 18h 21, 10h 21, 18h
Perigee 22, 11h 22, 9h 19, 5h
Solstice 22, 8h 22, 9h 21, 17h
What is really rare is that in 1999 the three events take place in
such quick succession. On only two other occasions in modern history
have the full Moon, lunar perigee, and December solstice coincided
within a 24-hour interval, coming just 23 hours apart in 1991 (as
indicated in the preceding table) and 20 hours apart back in 1866. The
10-hour spread on December 22, 1999, is unmatched at any time in the
last century and a half.
So is it really true, as numerous faxes and e-mails to Sky & Telescope
have claimed, that the Moon will be brighter this December 22nd than
at any time in the last 133 years? We have researched the actual
perigee distances of the Moon throughout the years 1800-2100, and here
are some perigees of "record closeness" that also occurred at the time
of full Moon:
Date Distance (km)
1866 Dec. 21 357,289
1893 Dec. 23 356,396
1912 Jan. 4 356,375
1930 Jan. 15 356,397
1999 Dec. 22 356,654
2052 Dec. 6 356,421
It turns out, then, that the Moon comes closer to Earth in the years
1893, 1912, 1930, and 2052 than it does in either 1866 or 1999. The
difference in brightness will be exceedingly slight. But if you want
to get technical about it, the full Moon must have been a little
brighter in 1893, 1912, and 1930 than in either 1866 or 1999 (based on
the calculated distances).
The 1912 event is undoubtedly the real winner, because it happened on
the very day the Earth was closest to the Sun that year. However,
according to a calculation by Belgian astronomer Jean Meeus, the full
Moon on January 4, 1912, was only 0.24 magnitude (about 25 percent)
brighter than an "average" full Moon.
In any case, these are issues only for the astronomical record books.
This month's full Moon won't look dramatically brighter than normal.
Most people won't notice a thing, despite the e-mail chain letter that
implies we'll see something amazing.
Our data are from the U.S. Naval Observatory's ICE computer program,
Jean Meeus's Astronomical Algorithms, page 332, and the August 1981
issue of Sky & Telescope, page 110.
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