CANADA: In colloquial terms, a “blue moon” is either a Belgian beer usually served with an orange, or an adept way to explain something that happens very rarely. In astronomical terms, a blue moon is actually the name for the second full moon that occurs in a single month.
We already had a full moon on July 2, so when a full moon rises again on Friday, July 31, it will indeed be a blue moon.
But don’t get too excited—the moon won’t actually be blue. It will look more pale gray and white, exactly like the moon you’re accustomed to seeing in the night sky. But, it’s still an astronomical event, as the last blue moon occurred in August 2012. After this one, the next one won’t appear until January 2018. Actually, there will be two blue moons that year, the second in March. After that, there won’t be two blue moons in a year again until 2037.
According to a new video from NASA, the modern definition for a blue-moon-that-is-not-really-blue came about in 1946. Writing in Sky & Telescope aiming to explain blue moons to the layman, author James Hugh Pruett cited the 1937 Maine almanac, saying “the second [full moon] in a month, so I interpret it, is called Blue Moon.” This was actually incorrect, but the name and definition stuck.
That’s not to say there has never been a real blue moon.
The moon can appear blue in color if enough smoke or dust particles enter the atmosphere—but they must be slightly wider than 1 micron, or roughly the same as the wavelet of red light. According to NASA, particles of this special size strongly scatter red light, while allowing blue light to pass through. The smoke essentially acts as a blue filter.
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