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Once in a Blue Moon

Week of January 11, 1999 The phrase "once in a blue moon" is a common one in English. It is used to mean a very rare phenomenon, like, "I get a raise once in a blue moon." But what is a blue moon, and how rare is it?

As it happens, a blue moon is not all that rare. A blue moon is not really blue, but actually is the name given to the second full Moon in the same month. It takes the Moon about 29.5 days to go from full through all its phases and back to full again. Since that is less than the days in an average month, it's possible to have two full Moons in one month (if the first one is early enough in the month).

Note added January 12, 1999: I have just been informed by a reader that the phrase "blue moon" has been around for centuries, but the meaning to which I attribute it is actually only a few years old! For more info, read the Wikipedia entry for Blue Moon.

Since the month as we measure it and the lunar cycle are not exactly equal, the full Moon falls on a different day each month. But after a while the two cycles match up again, so that if you wait long enough, the phases will fall once again on the same day. That period is about 19 years, so if a full Moon happens on January 2 of one year, it will be on January 2 19 years later. This period has 236 full Moons in it, and 228 calendar months. The difference between those two is eight, so that there must be eight months with two blue moons in them over that period! Since 8/228 = 0.035, about 3.5 percent of all months have blue moons. It isn't so rare as we think! [Oops!(Feb. 23 1999): This isn't quite true. Since February has 28 days, sometimes it doesn't have a Full Moon at all, like in 1999. So actually there are at least 8 Blue Moons in the cycle. Thanks to Reader Henk Metselaar for pointing this out to me.]

Oddly, a funny pattern can creep in, which is why I bring this up now. If a blue moon happens in January, it means that the second full Moon happens very late in the month. If it isn't a leap year, then February has 28 days, and it's possible that February gets no full Moon at all! Odder still, the full Moon will occur early in March, which in turn means March gets a blue moon as well as January, while February doesn't even get a full Moon! That happens this year. The first full Moon was on January 2, and the blue moon on the 31st. There is no full Moon in February at all, and then March has two, one on the 2nd and the other on the 31st. That pattern repeats every 19 years! So you'll have to wait until 2018 to see it again.

For more info about this, plus a blue/full Moon calculator, try out Blue Moon page which is part of Obliquity. You can see when the full Moon occurs and get lots of more information about the lunar cycles.

©2008 Phil Plait. All Rights Reserved.

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