This month, the December full moon falls on Friday, December 25, 2015. For Earth’s Western Hemisphere, it’s the first full moon on Christmas Day since 1977.
We won’t have another full moon on a Christmas Day until 2034.
A 19-year cycle of the moon is the reason. Amazingly, the moon’s phases recur on (or near) the same calendar dates every 19 years. This cycle – known as the Metonic cycle – happens because 235 returns to full moon almost exactly equal 19 years.
So, in other words, the phases of the moon realign (or nearly realign) with the same calendar dates every 19 years. We just missed a full moon on Christmas 19 years ago; instead, the full moon fell on Christmas Eve. It was December 24, 1996 at 20:41 Universal Time, or UT.
But two Metonic cycles ago – 38 years (or 2 X 19 years) – the full moon fell on Christmas Day. That full moon happened on December 25, 1977 at 12:49 UT.
Astronomically speaking, the moon is only full for an instant – at the moment that it’s 180o opposite the sun in ecliptic longitude.
This month, that happens on December 25, 2015 at 11:11 UT. At United States time zones, that translates to 6:11 a.m. EST, 5:11 a.m. CST, 4:11 a.m. MST or 3:11 a.m. PST. However, if you sleep through it, don't fret.
So if you love Christmas, and you love the moon, this is your moment!
To some, gazing at the moon may seem mundane, but the biggest and brightest object in our night sky continues to surprise scientists.
Just this week, researchers using data collected by China’s Chang’e 3 rover revealed evidence of a new type of basaltic rock in one of the dark basins on the moon. The finding suggests the lunar surface is more diverse than the one that emerged from studies following the Apollo and Luna missions.
In 2014, a different group of scientists found that volcanoes may have erupted on the moon during the time dinosaurs roamed the Earth -- practically last week by geological time scales.
And consider this: Even after more than a few hundred thousand years of staring up at the moon, we humans are still not 100% sure how it was created.
"It's worth remembering that the moon is more than just a celestial neighbor," John Keller of NASA's Goddard Space Flight Center said in a statement. "The geologic history of the moon and Earth are intimately tied together such that the Earth would be a dramatically different planet without the moon."
And finally, we're leaving you lunar fans with a little holiday present from NASA. The space agency's Lunar Reconnaissance Orbiter captured this composite image of the Earth in all its watery glory, as seen against the dark, undulating surface of the moon.
Day and night sides of Earth at the instant of December 2015 full moon (2015 December 25 at 11:11 Universal Time). The shadow line running south of North America and north of South America indicates sunrise December 25. The shadow line running to the east (right) of India depicts sunset December 25.
Moon cycles are not commensurate with the calendar, which is based upon the seasons – not the lunar cycle. Therefore, one year from now, the December 2016 full moon will not fall on December 25.
In any year, the phases of the moon take place about 11 days earlier than in the previous year. For instance, the December 2016 full moon will happen on December 14, 2016, and the December 2017 full moon will fall on December 3, 2017.
The moon will look plenty full tonight as it lights up the nighttime from dusk till dawn. Look for the full-looking moon to rise opposite of the setting sun at early evening. The moon will shine highest up for the night around midnight and then will sink low into the western sky as dawn colors the sky.
For most of the world’s Eastern Hemisphere (Asia, Indonesia, Australia and New Zealand), the last Christmas full moon happened in 1996. In 1996, the full moon occurred on December 24 at 20:41 Universal Time … December 25 in those places.
Bottom line: The December 25, 2015 full moon is the Western Hemisphere’s first full moon on Christmas Day since 1977. We won’t have another full moon on a Christmas Day until 2034. A 19-year cycle of the moon – called the Metonic Cycle – is the reason. Explanation here.
Happy holidays and happy sky watching!
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