Alachua Astronomy Club: December 1999's Bright Full Moon

	Alachua Astronomy Club, Inc.	Member Astronomical League	Member International Dark-Sky Association
	December 1999's Bright Full Moon	Member Astronomical League	Member International Dark-Sky Association
	29°39' N, 82°21' W Altitude: 50 Meters (more or less)		Updated April 3, 2000
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1999's "Moon Before Yule": Brightest Full Moon in 133 Years?

How bright I am!
(Percentage of light Moon reflects?)
Click for answer
Recently "Moon gossip" has been spreading throughout the Internet, the news media and on such programs's as Jack Horkheimer's weekly TV program, Star Gazer. (See Episode #99-50.) According to Sky & Telescope's News Bulletin, this moon talk may have started because of an article in the Old Farmer's Almanac.
(Other full Moon stories below.)

It goes something like this:

This year will be the first full Moon to occur on the winter solstice, December 22, commonly called the first day of winter.
This full Moon will also be the brightest in 133 years.
Furthermore, this year the winter solstice occurs nearly at the same time as the lunar perigee (point in Moon's orbit closest to Earth).
Thus, the Moon will appear about 14% larger than it does at apogee (point in Moon's orbit farthest from Earth).
Since the Earth is also several 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 appear brighter.
Also, this will be the closest perigee of the Moon of the year. (The Moon's orbit constantly "deforms" so the perigee distance is not constant.)
So, this December's full Moon will be a super bright full Moon, much brighter than the usual and it hasn't happened this way for 133 years! If the weather is clear and there is a snow cover where you live, it is believed that even car headlights will be unnecessary. The last time this happened, on December 21st, 1866, the Lakota Sioux took advantage of the exceptionally bright moonlight and staged a devastating retaliatory ambush on soldiers in the Wyoming Territory.

Moon Madness! Moon Mishugaas! Moon Hooey! — Much of This is Rubbish!

Yes, 1999's December full Moon, sometime called the "Moon before Yule," occurs less than ten hours after the winter solstice. And, yes, this full Moon occurs several hours after the Moon reaches perigee, which does happen to be the closest perigee of 1999.
However, to put this moon business in proper perspective, note the following:

This is not the first full Moon to occur on the Winter Solstice. It periodically occurs about every nineteen years. The last one was in 1980. And it happened in 1961 and 1942, etc.
(Note: The concurrence of two such events on the same date is time zone dependent so sometimes one event may fall on the following or preceding date.)
Even the near simultaneous occurrence of the full Moon, winter solstice and lunar perigee is not all that rare. For example, it also happened in December 1991 and December 1980.
Although the December 1999 lunar perigee is closest for 1999, the full Moon and the lunar perigee have occurred almost simultaneously several times in the last 133 years including 1866, 1893, 1912, 1930 and 1999. (Another will occur in 2052.)
In fact, all of these five occurences of full moons and perigee distances put the Moon closer to the Earth than the December 1999 event! (The 1912 event was closest of all.)

So, the December 1999 full Moon is not the brightest full Moon in recent times!

But is there anything unusual or interesting about the December 1999 full Moon? Yes there is. According to Roger W. Sinnott, associate editor of Sky & Telescope magazine, what is unusual about this full Moon is to have lunar perigee, solstice, and full Moon occur in such quick succession—a spread of only ten hours. Furthermore, he writes, 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 and just 20 hours apart back in 1866. Thus, Sinnott concludes, the 10-hour spread on December 22, 1999, is unmatched at any time in the last century and a half! [Sky & Telescopei News Bulletin for December 17, 1999]

How Much Brighter is a Really Bright Full Moon Than "Average"

Not much. Not enough that anyone will really notice.

Using the distance from the Earth-Moon and the distance of the Earth-Moon System from the Sun for the December 22, 1999 event, I calculated the Moon's increase in brightness compared to an "average" full Moon:

The lunar perigee effect should increase the Moon's brightness about 20%.
The nearness of the Earth-Moon System to the Sun should increase the Moon's brightness about 4%.
The net increase is about 25%.
For those versed in astronomical magnitudes, these changes amount to about a 0.20 and 0.04 mag. increase respectively, or a net increase of only 0.24 mag.

This increase is too small to be easily noticed by most people!

(Note: My calculations are consistent with those done by Jean Meeus, Sky & Telescope, August 1981, pp.110-111.)

[For more information on the Moon's perigee see, Jean Meeus, Mathematical Astronomy Morsels, 1997 (Willman-Bell). For more material on bright full moons see Sky & Telescope News for December 16, 1999.]

But There Is Still More!

There is another effect that often makes all winter full moons seem extra bright!

Winter full moons in Earth's Northern Hemisphere ride high during the night! A full Moon is opposite the Sun's position on the sky and does everything "opposite." In a sense, the winter full Moon mimics the summer Sun. A summer sun (for most of the Northern Hemisphere) rises in the northeast, crosses high over the south horizon, sets in the northwest, provides more than twelve hours of daylight, and provides more direct sunlight. The winter full Moon does likewise. It is as if mother nature has provided us with an abundance of moonlight in the winter to make up for the lack of sunlight!

Add a crisp, clear winter night sky, and the full Moon will appear to swing high during the night making the night scene seem so much richer in light!

High, winter full moons often make us perceive winter full moons as extra bright—not the imperceptible difference due to the "perigee effect."

Finally, if a person should see the full Moon just rising above a clear, horizon sky, the "moon illusion" will also make one think the Moon is extra big! (The "moon illusion" is a psychological illusion that makes objects such as the Sun or Moon appear extra large when near the horizon.)

Conclusion: All Full Moons Are Bright!

And Still More: The First Full Moon of the Year 2000
Forget the December 1999 full Moon. The next full Moon (January 21, 2000) is even more special!

Like the December 1999 full Moon which occurs near the smallest lunar perigee for 1999, the January 2000 full Moon also occurs close to the time of the smallest lunar perigee for 2000!
Also, the Earth-Moon System will still be near its closest point to the Sun (January 3, 2000).
Hence, the January 2000 full Moon will be the largest and brightest of 2000!
Yes, the January lunar perigee is slightly farther than the December perigee and the January full Moon occurs about a day before full Moon.
Still, the January 2000 full Moon will be nearly as bright as the December 1999 full Moon (about 18% brighter than an "average moon" rather than 25% for the December 1999 full Moon).
This translates into a January full Moon only 0.05 mag. less bright—an imperceptible difference!

So what makes the January 2000 full Moon extra special?

Answer A total eclipse of the Moon that will be visible nearly all night long for observer's in the USA!

The lunar eclipse occurs on the night of Thursday/Friday January 20/21, 2000:

(Times given are Eastern Standard Time, which is five hours later than Universal Time)
Moon enters penumbra (outer, lighter part of Earth's shadow) 9:03 p.m. EST
Photo by David Muir

Moon enters umbra (inner, darker part of Earth's shadow) 10:02 p.m. EST

Moon fully immersed in umbra 11:05 p.m. EST

Greatest eclipse 11:44 p.m. EST

Moon begins to leave umbra 12:22 a.m. EST

Moon fully out of umbra 1:26 a.m. EST

Moon leaves penumbra 2:24 a.m. EST

This total lunar eclipse may be more spectacular than average:

The entire eclipse is visible for the continental USA not just the beginning or end as sometimes happens.
It is a winter eclipse—the moon will "ride high" during the night, hopefully against a crisp winter sky.
The north limb (edge) of the Moon will near the center of the Earth's shadow while the south limb will be fairly close to the edge of the Earth's shadow possibly allowing a strong gradation of light (and color) across the face of the Moon.
(Colors and darkness can range from an almost invisible Moon near mid-eclipse for some lunar eclipses, through brownish, reddish and even bright, copper-red or orange for other lunar eclipses.)
The Earth's atmosphere is now relatively clean of volcanic dust which may produce a brighter than normal eclipse.
(The Moon often does not completely "disappear" during totality due to light from the Sun which the Earth's atmosphere scatters into the shadow of the Earth.)
Hence, each lunar eclipse can appear different and the Moon's unpredictable appearance adds to the excitement of lunar eclipses!

Want to photograph this eclipse? Here are details from the New York Institute of Photography.

Other Stories About The December 22, 1999 Full Moon

For a newspaper story by Gainesville Sun staff writer Carrie Miller about the "brightestet-mooon-of this-century," see SunOne for Tuesday, December 21, 1999.

Astronomy Picture of the Day for December 22, 1999 features perigee and apogee Moons.

Sky & Telescope News for December 15, 1999 discusses "Brightest Moon in 133 Years?"

(Times given are Eastern Standard Time, which is five hours later than Universal Time)
Moon enters penumbra (outer, lighter part of Earth's shadow)	9:03 p.m. EST	Photo by David Muir
Moon enters umbra (inner, darker part of Earth's shadow)	10:02 p.m. EST
Moon fully immersed in umbra	11:05 p.m. EST
Greatest eclipse	11:44 p.m. EST
Moon begins to leave umbra	12:22 a.m. EST
Moon fully out of umbra	1:26 a.m. EST
Moon leaves penumbra	2:24 a.m. EST

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