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Blue Moon |
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A "Blue Moon" is the second full moon in a calendar month. At least, that is the definition that I have always understood, and apparently so have a lot of other people. However, this is not the original definition of the term "Blue Moon". Much like language and the meaning of words which can evolve to give us new words and new meanings of words, acceptable definitions can also evolve and become the new norm. The original definition of a Blue Moon was that it was the third full moon in a season that has four full moons. This is the definition of what is referred to as a seasonal Blue Moon which occurs about every 2.5 years, according to NASA. There are roughly 29.5 days between full moons, making it unusual (but possible) for two full moons to occur within a 30- or 31-day-long month. This also means that February, even in a leap year when February is 29 days long, will never have a Blue Moon. There are 92 days in the spring season, 93 in the summer, and 90 days in each of the fall and winter seasons. Dividing each of these days-per-season numbers by 29.5, we get values that are all greater than 3.0 meaning that all of the yearly seasons could have a (seasonal) Blue Moon.
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The next monthly Blue Moon (a second full moon that appears in a single month) will be on October 31, 2020 (according to NASA), while the last monthly Blue Moon was on March 31, 2018. There was a seasonal Blue Moon on May 21, 2016 and the next one will be in the spring of 2019 on May 18. The term "Blue Moon" has nothing to do with the actual color of the moon. Sometimes conditions in the earth’s atmosphere will make the color of the moon appear red or blue or some other color, but most Blue Moons look pale gray and white, indistinguishable from any other moon. Squeezing a second full moon into a month does not change the properties of the moon itself, so its color remains the same. The rare occurrence of a Blue Moon has given rise to the phrase "once in a Blue Moon" which has been around for more than 400 years, according to one historian, who likened the use of the term to that of saying the moon is made of green cheese: it indicated something absurd. The meaning evolved to something like saying "never". . ."I’ll marry you when the moon turns blue".
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