Christmas full moon

(A bit late, sorry!)

The moon was full this Christmas, for the first time in 38 years.  This garnered some media coverage – see for example CNNVox, and Forbes.  The next one will be 19 years from now.  (The available data on this seems to be from NASA and generally is based on US Eastern time – your mileage may vary in other time zones.)

What would we expect? In the long run, since there’s a full moon every 29.5 days, a full moon on any given calendar date should happen every 29.5 years.  But in the medium run it appears that the Metonic cycle kicks in – that is, the fact that 235 lunar months is very nearly equal to 19 solar years, so full moons are on nearly the same calendar dates in years nineteen years apart.  Looking at this list of full moons from 1900-2100, there are full moons at (all times Eastern US):

1901 Dec 25 07:15

1920 Dec 25 07:39

1939 Dec 26 06:29

1958 Dec 25 22:54

1977 Dec 25 07:50

1996 Dec 24 15:43

2015 Dec 25 06:13

2034 Dec 25 03:56

2053 Dec 25 04:25

2072 Dec 25 02:18

2091 Dec 25 12:02

and nine of these eleven fall on Christmas.  (The pattern is a bit irregular – the number of hours between full moons isn’t quite constant.) The previous cycle was one of the times when the full moon missed Christmas.

This cycle also comes up in the computation of Easter, which is nominally on the first Sunday after the first full moon after the vernal equinox.

More if I can find some code to compute the time of full moon…

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