The Great Christmas Full Moon of 2015!

Here is my take on this year's December 25 full moon. It's expanded from a paragraph that will be appearing in my "What's Up" Gettysburg Times column this month and includes some material from our December planetarium show. --Ian Clarke

One calendrical oddity that may get some press this holiday season is the full moon on Christmas Day, something that you will hear has not occurred since 1977 and will not again until 2034. If nothing else, it’s a great occasion to dig a little deeper into the moon cycle. 

The lunar cycle has been a bit troubling to human calendar makers, because at first blush it seems it might fit into the solar year. 12 lunar months make up about 354 days. Unfortunately, if you want to keep track of the seasons of the solar year, an 11-day annual error is a pretty big problem. Our Gregorian calendar is therefore a solar one, with a nod to the lunar cycle in the form of weeks and months. These approximate the moon's quarter and full phase cycle, but not in any way that would actually allow you to keep track of them. The actual phase cycle from one full moon to next is 29.53 days. An average month in our calendar is just under 30.44 days, so a given moon phase slips backward about nine-tenths of a day per month when averaged out over our uneven months.

If you are satisfied with the moon appearing roughly full on Christmas (or any other specific date), you don’t need to wait too long. Any full moon within two or three days will do. The moon was near full on December 25 in 2007 (full the 24th) and 2012 (28th) and will be again in 2018 (22nd) and 2023 (27th). 

If on the other hand, you are looking for the moment of full moon to fall exactly on a given date, you must learn about the metonic cycle. As we saw, 12 lunar cycles don't fit evenly into one solar year, but if you wait long enough, you can find a pattern. 235 lunar phase cycles is almost exactly 19 years (6,939.688 days vs.6,939.602 days). Pretty neat! It's called a metonic period. Does that mean a full moon will recur on a given date every 19 years? Almost. Because the match is not exact and because of our leap years, some 19 year intervals are skipped. We have not had a full moon on December 25 in 38 years, so we must have skipped 1996. Then the moon was full on December 24 at 20:41 Universal Time. It missed by a little over three hours. We won’t miss any metonic cycles in the 21st century, so see you, or your heirs, again on December 25, 2034, 2053, 2072, and 2091.

Here is a final complication. Astronomically speaking, the moon is counted as full only for an instant - in this case, December 25 at 11:11 Universal Time. That is 6:11 AM EST, so to see this Christmas full moon in eastern North America, you'll need to be up before sunrise. And in Europe it will be the middle of the day, and the moon, though full, won't even be up. Thinking along those lines, we earthlings did not exactly miss the 1996 Christmas full moon either. When the moon was full on December 24 at 20:41 UT, it was already December 25, local time, in Australia and much of the rest of the Eastern Hemisphere.