Dateline: 12/16/99
There's a email spreading around the 'Net faster than wildfire that claims that next Wednesday, December 22, there's a once-in-a-lifetime astronomical event not to be missed. You might have received something similar to this...
This year will see the first full moon to occur on the winter solstice in 133 years!In the Northern Hemisphere, December 22 is commonly called the winter solstice or the first day of winter. Since a full moon on the winter solstice occurs in conjunction with a lunar perigee (point in the moon's orbit that is closest to Earth) the moon will appear about 14% larger than it does at the lunar apogee (the point in its elliptical orbit farthest from the 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 brighter. Also, this will be the closest perigee of the Moon for this year as the moon's orbit is constantly deforming. If the weather is clear and there is a snow cover where you live, it is believed that even car headlights will be superfluous.
The last time this event occurred was on December 21, 1866. The Lakota Sioux took advantage of this combination of occurrences and staged a devastating retaliatory ambush on soldiers in the Wyoming Territory.
What this means:
1. A super bright full moon will appear.
2. It will appear larger than usual.
3. This has not happened in this manner for 133 years!
4. It will be a once in a lifetime astronomical event.
This story is mostly true. December 22, the solstice (the beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere) does coincide with the moon at perigee, its point closest to the earth. That means the moon will appear 14% larger to us than at apogee, when the moon is at its farthest from the earth. The moon will be a mere 356,654 km (221,614 miles) away at perigee.
However, this is not the only full moon on a solstice in the last 133 years, there was a full moon on the December solstice as recently as 1980. Nonetheless, this is the only perigee full moon in 133, since 1866. This moon will indeed be brighter and it should be a stunning event if the sky is clear.
The Old Farmer's Almanac does mention the story of the Lakota Sioux. As an interesting side note, the perihelion, the point when the earth is closest to the sun occurs on January 3, 2000.
For more information, visit the The Old Farmers Almanac and Sky & Telescope articles about this topic.
Visit the About.com Space site for more information about the earth's natural satellite, the moon.
