Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
December 1999

Next Wednesday, December 22, the northern hemisphere will witness a rare conjunction of events. At 1:44 a.m. CST the Earth reaches winter solstice, the first day of winter when the Sun is at its southernmost extent in the sky. At 4:55 a.m. CST the Moon reaches perigee, the point in its orbit where it is closest to the Earth (at a distance of 221,614 miles, about 7% closer than its average distance). Then at 11:31 a.m. CST the moon is completely full, the so-called Cold Moon (December's full moon, also known as the Oak Moon, the Long Night Moon, or the Moon before Yule).

This combination of full moon at perigee on the winter solstice only occurs about once each century. The last time it happened was 133 years ago.

The full moon at perigee will appear about 14% larger (in surface area) than normal and will be about 7% brighter than the average full moon. Some sources claim that if you add snow cover on the ground, headlights will be unnecessary.

The full moon on the winter solstice means the moon will rise and set well north of due east or west. Additionally, since this is the shortest day (and thus longest night) of the year for the northern hemisphere, we'll have the longest "moon night" possible.

Don't miss this special occurrence. You probably won't be around for the next one!


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Last update:  December 17, 1999