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I am sure you’ve heard of the popular phrase, “once in a blue moon” many times and probably even know its metaphoric meaning: a rare phenomenon or something that seldom happens.
Today is such a blue moon, something that rarely happens…
Or literally you might even be tempted to think that it’s when the moon color appears blue in the sky and you won’t be wrong either! A blue moon is a phenomenon whereby the moon appears bluish due to certain sized smoke or dust particles in the atmosphere creating unusual sky conditions. For example, people saw blue moons for years after the 1883 eruption of Krakatoa. Blue colored moons aren’t predictable though.
But today’s blue moon is not a blue colored moon that appears in the night sky. There is yet another connotation of the blue moon. It is when the full moon appears twice in the same calendar month. This month, March 2018 the last full moon occurred on 2 March, Holi-day, which is an Indian festival of colors. And today, 31 March is the second full moon in the same month. So the second full moon in a month is called the Blue moon, not the first.
And such an occasion isn’t frequent; it happens approximately once in every 2.7 years. The next time such a thing occurs will be in 2020. Yet this is the second blue moon this year, the first having occurred on 31 January. This is rarer to have two blue moons in the same year, that too within a period of three months. So, what accounts for such an extraordinary occurrence? It takes approximately 29½ days for the moon to become full since the last phase of full moon. As February is the shortest month of the year, this year it did not see a full moon. Hence, March is seeing two full moons! This is why we have two blue moons this year within a period of 60 days, since January. However, tonight’s blue moon will be the last this year and next year 2019 we won’t see a single blue moon. We have to wait until Halloween 2020 for the next occurrence. Fascinating, isn’t it?
That said, earlier a ‘Blue Moon’ signified the third full moon in a season of four full moons. According to that definition, we would see a blue moon next year on May 18, 2019, which will be the next seasonal blue moon (third of four full moons in one season). A single season, each of winter, spring, summer and fall, usually has three full moons. If a season gets four full moons which is infrequent, then the third full moon is called a blue moon, according to the old Maine Farmer’s Almanac.
So this sense of an “extra” full moon transmuted into the definition of a blue moon that most dictionaries define today, which is the second full moon in a single calendar month. Oh language is forever metamorphosing, mutating, becoming something else! So anyways, let’s stick with this new definition for now, shall we?
The last blue moon on 31 January was rare because it was also a supermoon and a total lunar eclipse as the full moon passed directly through the earth’s shadow! A supermoon is when the moon appears bigger in the sky due to its proximity to earth, as full the moon is closest to earth (perigree). The last time such a thing happened over North America was in 1866! Not only that, January’s blue moon actually appeared reddish, like a coppery color, owing to atmospheric refraction, hence also calling it the “blood moon”!
So that was January. But what is special about today’s full moon? Well, March’s blue moon is also a Paschal moon for Easter. A Paschal moon is the first full moon after spring dawns in March, and the first Sunday after the Paschal moon is when Easter falls usually, which is tomorrow April 1. The last time Easter Sunday fell on April 1 was in 1956!
A Paschal moon which is also a blue moon can only occur in March. The last time a Paschal blue moon occurred was in March 1999 and the next time a Paschal blue moon will appear in March 2037… quite a wait, huh? So when was the last time we had a blue Paschal moon on March 31, followed by Easter Sunday, the very next day?
Well it was in 1714, but only for Europe and the East because for the West the full moon happened a day before on March 30 owing to the time difference. If we are to look at North America, it was back in the year 1646 when a blue Paschal moon occurred on March 31, followed by Easter Sunday the following day. However, that time the original colonies were still following the Julian calendar, and not the Gregorian calendar of today. And although the Gregorian calendar got implemented in 1582, England and the American colonies switched to it only in 1752; thus this year’s Easter would be inaccurately calculated as it would put everything off by 10 days. Therefore, you see tonight’s Paschal blue moon with Easter Sunday the following day on April 1, is truly unique, one of its kind!
So, today the full moon is indeed special, a true blue moon! Be sure to watch out for it in the nite sky. And what if it also appears blue in color? Well that would be double reason to celebrate, but don’t let the blue moon make you blue!
Oh well, that’s just another phrase with blue! This thing called Language… I tell ya!
Resources: space.com, earthsky.org
The blue moon on 31 March 2018, from my camera. Click on the photos to enlarge. Does it look bluish? hahaha
baisakhi saha 
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