Wednesday, December 15
Full moon at Perigee and Apogee.
Suddenly a lot of people are asking this question: Will the full Moon of December 22, 1999, 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:
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:
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 just one day after 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.
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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.
© 1999 Sky Publishing Corp. All rights reserved.
Webbed by Philip McEldowney
Last update: Thursday, 23-Dec-1999 12:27:37 EST.