Why are there so many blue moons in February?

The traditional blue moon is the third full moon in a season having four in total. This happens seven times every nineteen years. From Wikipedia, I found that the spring and summer seasons are considerably longer than autumn and winter, and winter, from the solstice to the equinox, is only 88.99 days long. Since three lunar cycles are 3 x 29.53 = 88.59 days, there must be a full moon in the first ten hours (0.40 days) for there to be a February blue moon 59 days later. I calculate (29.53 / 0.4) that this should occur about once every 74 years; 85% of blue moons should be in May (spring) and August (summer). But another website shows quite a few more February blue moons in the 20th and 21st centuries than I'd expect. Why?

Am looking for an answer that mentions equinoxes, solstices, and the lunar month.


Bikenbee has the best answer. The website is http://www.obliquity.com/astro/seasonal.html

and by looking at it more carefully, I've found the answer myself. The Maine Farmers Almanac used seasons of equal length, and that gave a roughly equal distribution of seasonal blue moons. But by using actual seasons, there are only 3 winter (February) blue moons in 200 years (20th & 21st centuries), and 88% of the blue moons occur in May (spring) and August (summer). All of this agrees with the calculations I made before posting this question -- a February blue moon every 74 years, with 85% of blue moons occurring in May and August. Blue moons in 200 years are February (winter) 3, May (spring) 29, August (summer) 37, November (autumn) 6. So "once in a blue moon," when referring to autumn & winter, is especially rare -- 9 times in 200 years!

If you want to comment, Bikenbee, I'll hold off the "best answer" award for 24 hours.

3 Answers

  • 1 decade ago
    Favorite Answer

    The Earth reaches perihelion in early January, when it's orbital speed is the fastest, so the Moon has to do some catching up and the average interval between full moons in the early part of the year is likely to be slightly more than 29.53 days, making the February blue moon (according to your definition) even rarer than you calculate. However, I agree with your calculations in general. Perhaps you could indicate which website you are disagreeing with.

  • 1 decade ago

    First up, you need to determine if you are using the traditional or modern Blue Moon definition. This will likely affect the answers that people will craft. More clarification please.

    Secondly, Spring here is 1st September, 1st March is Autumn, so you're probably best to reference months and not seasons. Summer solstice here is December 21st; how about you?

  • 1 decade ago



    only the 'old' kind show in Feb... that month is too short to hold the 'new' type....

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