What is a Blue Moon?

Once in a blue moon is a common way of saying not very often, but what exactly is a blue moon? It is the third full moon to occur in a season, which has four full moons in it. Now you may have heard that a blue moon is the second full moon in the same month. However, that is a bad definition from a misinterpretation of what a blue moon is. For a full explanation, read on...

In March 1999, Sky and Telescope magazine published an article about blue moons by Philip Hiscock, who has studied the folklore and history of the expression. In that article, Hiscock traced the many meanings of the expression over the centuries, but noted that the "two Full moons in a single month" meaning couldn't be explained satisfactorily.

In the May 1999 issue of Sky and Telescope, there appeared a follow-up article, which proved that Sky and Telescope had in fact created the current meaning by mistake in an article published in March 1946. The author of the 1946 article had misinterpreted a page of the 1937 Maine Farmers' Almanac. By studying copies of the Maine Farmers' Almanac dating as far back as 1819, the authors of the May 1999 article showed that the compilers of the Almanac used the term to label the third full moon in a season which has four.  The added benefit to using the original system is that you have the blue moon everywhere in the world in the same month. Taking the new definition and using a blue moon for 2001, we see that there is one in November.

2 October at 13:49 GMT
1 November at 05:41 GMT
30 November at 20:49 GMT
30 December at 10:40 GMT

These dates and times have been calculated rigorously using the same methods as those used by the United States Naval Observatory and by Her Majesty's Nautical Almanac Office. They are given in Greenwich Mean Time (GMT) which is the standard time zone used by astronomers worldwide. Notice that the second Full Moon in the list occurs just 5 hours and 41 minutes after the start of November, and the third Full Moon is just 3 hours and 11 minutes before the end of the month. In either case, changing the time zone by only a few hours will change the month in which the Full Moon falls.

So, go and amaze your friends with this wonderful information...or just forget it and go buy some comics.