• There are a number of different definitions of a blue moon, the most accepted being the 13th full moon in a year, occurring once every two to three years. The Farmers’ Almanac follows the rule of the extra (fourth) full moon in a season, and the definition of the blue moon as the second full moon in one month stems from a 1946 interpretation error, revealed in the May 2009 issue of Sky & Telescope. (In case you missed the May issue, here’s the article.)

• The moon before the blue moon is called a betrayer moon, because it was seen as coming ahead of its appropriate time (the blue moon was one appearing at the right time, then?) and thus confusing the clergy when they were trying to determine the dates for Lent and Easter, which apparently were based on the dates of the full moon.

• An interesting list of the traditional English names (e) for full moons, based on the Gregorian calendar, followed by the Native American (na) names, based on an older method of dating full moon according to seasons:

January: old moon (e), wolf moon (na)
February: wolf moon (e), snow moon (na)
March: lenten moon (e), worm moon (na)
April: egg moon (e), pink moon (na)
May: milk moon (e), flower moon (na)
June: flower moon (e), strawberry moon (na)
July: hay moon (e), buck moon (na)
August: grain moon (e), sturgeon moon (na)
September: corn moon (e), harvest moon (na)
October: harvest moon (e), hunter’s moon (na)
November: hunter’s moon (e), beaver moon (na)
December: oak moon (e), cold moon (na)

• Finally, here is a handy equation you can use to calculate the dates of future full moons (all full moons, not just blue moons):

where d is the number of days since 1 January 2000 00:00:00 in the time scale known as Terrestial Time. For Universal Time (world clock time) add the following approximate correction to d:

days

and where N is an integer number of full moons, starting with 0 for the first full moon of the year 2000. The true time of a full moon may differ from this approximation by up to about 14.5 hours as a result of the non-circularity of the moon’s orbit.

The next time the blue moon will fall on New Year’s Eve is 2028, at which time there will also be a full lunar eclipse! Whoa! (I did not calculate this myself, although I feel like we should test this equation to make sure mathematician Jean Meeus is correct.)