Last Blue Moon Until 2015 to Light Up Tonight’s Sky
Skywatchers last chance to to observe a celestial phenomenon for nearly three years is tonight, Friday, August 31, 2012. But don’t expect to see a glowing blue ball in the sky, as “blue moon” does not refer to color, but rather to rarity.
The moon will wax to its full phase at 9:58 a.m. EDT (1358 GMT) today, bringing August’s full moon count to two (the first one occurred Aug. 1). Two full moons won’t rise in a single month again until July 2015.
The term has long been used to describe rare or absurd happenings. Blue moons are best known from the phrase “once in a blue moon,” meant to refer to something extremely rare.
Blue moons are defined as either the fourth full moon in a season, or, more recently, as the second full moon in a month. In traditional folklore, blue moons were defined as the third full moon in a season with four (three being the norm). But that definition shifted in 1946, when a writer for Sky and Telescope magazine simplified the definition (or got it wrong, depending on your perspective) to the two-in-a-month phenomenon. The simpler definition caught on.
Blue moons occur because our calendar months don’t precisely sync with the moon’s orbit. It takes the moon 29.5 days to wax and wane from full to new to full again. With the exception of February, months are longer than that, meaning that once in a while the timing works out so there are two full moons in a single month.
Two blue moons in a month aren’t all that unusual, however; they occur 2.7 years apart on average, though not with great regularity. The last blue moon occurred on Dec. 31, 2009. That “New Year’s Eve Blue Moon” also coincided with a partial lunar eclipse for viewers in Europe, Asia, Africa and some parts of Alaska. Another rare event is a year with two blue moons. The last time a single year had two months with two full moons was in 1999. The next time double blue moons will occur is in 2018.
The moon will be at its fullest at 9:58 a.m. EDT (1358 GMT) on Friday.
When are the next full moons?
2015 July 31
2018 January 31 and March 31 (no February full moon)
2020 October 31
2023 August 31
2026 May 31
2028 December 31
2031 September 30