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.
Although the moon turns full at the same instant worldwide, the clock reads differently by timezone. On a worldwide scale, the full moon actually comes at all hours around the clock. See the worldwide map below. The full moon takes place before sunrise December 25 in North America, noon in Europe and Africa and after sunset December 25 in eastern Asia, Australia and New Zealand.
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.
Source…..www.earthsky.org
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