The upcoming Dec. 21 full moon — besides distinguishing itself from the others in 2010 by undergoing a total lunar eclipse — will also take place on the same date as the solstice (the winter solstice if you live north of the equator, and the summer solstice if you live to the south).
Winter solstice is the shortest day of the year in the Northern Hemisphere and marks the official beginning of winter. The sun is at its lowest in our sky because the North Pole of our tilted planet is pointing away from it.
So, how often does the December full moon coincide with the solstice? To answer this question, let's use Universal Time (UT), also sometimes referred to as Greenwich Mean Time (GMT). We do this because in answering this question, it's important to define a specific time zone.
For example, if you live in Honolulu, this December's full moon does not fall on the date of the solstice. Hawaii Time runs 10 hours behind GMT and the full moon occurs on Dec. 20 at 10:13 p.m. local time, while the solstice comes the following day at 1:38 p.m. Alaska, too, will have the full moon and the solstice occur on these respective dates, but in a time zone one hour later than Hawaii.
Prior to this year, there were solstice full moons in 1999 (Dec. 22) and 1980 (Dec. 21).
Interestingly, after this year, we'll have a long time to wait until we have a December full moon occur on the same date as the solstice: Dec. 21, 2094! And even more interesting – just like this year – that same full moon will fall into Earth's shadow in a total lunar eclipse. However, unlike this year, the 2094 eclipse will not be visible from the Western Hemisphere, but will be able to be seen from Europe, Africa and much of Asia. [How to Watch the Dec. 20 Total Lunar Eclipse]
Finally, this raises the question — prior to 2010, when was the last time that we had a total lunar eclipse occur on the same calendar date as the winter solstice? The answer, incredibly, takes us back nearly four centuries.
On Dec. 21, 1638, the full moon was in total eclipse from 1:12 to 2:47 UT. And the solstice occurred later in the day at 16:05 UT. [Amazing photos of a total lunar eclipse]
Once again, it's important to note that this occurred at the Greenwich meridian. For the Americas, this eclipse actually occurred during the evening of Dec. 20, while the solstice occurred on the following day.