Merry Christmas! In keeping with my most common theme for the holiday, I present a space story for Christmas. From Space.com: Rare Christmas Full Moon a Cosmic Gift for the Holidays: What to Expect.
If you live anywhere east of the Mississippi River this year, you likely will not have snow on the ground on Christmas morning, but you will have a full moon. The moon will officially reach peak fullness at 6:11 a.m. EST (1111 GMT). A full moon coinciding with Christmas does not happen often — usually, once every 19 years. But this time, it is different, because a full moon on Christmas hasn't happened in nearly four decades. The last one was in 1977, and that's because of the vagaries of our calendar and a lunar cycle known for almost 2,500 years.More science, history, and culture at the link, including full moons on Christmas in literature and popular culture.
The Metonic cycle is the lunar cycle in question. After 19 years, the lunar phases are repeated on nearly the same calendar dates. It was discovered around 432 B.C. by the Greek astronomer Meton (although some believe that the Babylonians knew about it before he did). After 19 years, the lunar phases are usually repeated on the same calendar dates.
According to the cycle, a Christmas Day full moon should have happened 19 years ago, in 1996, but it didn't because the Metonic cycle is only approximate and the number of leap days (four or five) in a period of 19 years can give rise to a discrepancy of one day.
In 1996, the full moon fell not on Christmas Day, but Christmas Eve. And in this particular case, adding 19 years to 1996 saw a jump of one day, moving the full moon to Christmas Day in 2015.
Interestingly, if we use the Metonic cycle going backward from 1996, we also jump forward one day to Christmas Day in 1977, which was the last time we had a full moon on Christmas Day.
Now, if we add 19 years to 2015, we find that the Metonic cycle will work perfectly with no one-day jumps — so the next time a full moon will occur on Christmas Day will be in 2034.
Finally, I'm sharing Season’s Greetings from NASA.
Mars is not the only place to explore during holidays. Follow our little Robot as he surveys his new surroundings. This holiday house is full of NASA treats… see if you can spot them all.Once again, Merry Christmas!