Once in a blue moon, as the saying goes, there are two full moons in a single calendar month, which NASA says actually happens every 2.5 years. A blue moon on New Year's Eve is much rarer, last happening in 1990. Even rarer, though, is the lunar eclipse that will accompany the blue moon.
Not only is the decade going to end on a blue moon -- the second appearance of a full moon in a calendar month -- but also a partial eclipse of the moon, at least for sky watchers in Europe, Africa and Asia. A partial lunar eclipse occurs when the moon is partially darkened by the Earth's shadow.
Most months, of course, have only one full moon but the 29.5-day lunar cycle occasionally forces a second full moon into the 28-to-31-day length of calendar months. The last time a blue moon occurred on New Year's Eve was 1990.
In fact, blue moons are not all that rare. On average there will be one blue moon every 2.5 years. What is rare is a partial lunar eclipse on New Year's Eve, it didn't happen at all in the 20th Century and the phenomenon will not occur again on New Year's Eve again until 2028.
The partial eclipse will be visible throughout Europe, Africa, the Middle East, Asia, Eastern Brazil and Australia. Some parts of Canada and the U.S. will also get to see a glimpse of this eclipse, but overcast skies are expected over most the U.S.
As for the origin of the term blue moon, NASA says the modern definition of came back in the 1940s when the Farmer's Almanac of Maine offered a definition of blue moon so confusing that even professional astronomers struggled to understand it. It involved factors such as the ecclesiastical dates of Easter and Lent, and the timing of seasons according to the dynamical mean sun.
Attempting to make some sense of what a blue moon means to layman, Sky & Telescope published an article in 1946 entitled "Once in a Blue Moon," which lay the basis for the modern definition.
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