The typical definition of a Blue Moon is the second Full Moon in a calendar month. We last had one of those back on August 31st, 2012, and the next one won't be until July 2015.
That's the modern definition, though. If we reach back further in time, the original definition of a Blue Moon says it's 'the third Full Moon in a season with four Full Moons.' For Summer 2013, we just happen to have four Full Moons — on June 23rd, July 22nd, August 20th and September 19th. With tonight being the third of the season, it gets to be included as a Blue Moon.
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Now, one thing to note is that the colour of the moon is going to depend on where you live. The moon can actually look blue, if there's been a recent volcanic eruption in the area (like in southern Japan on Sunday). This Science@NASA video from last year's Blue Moon gives a great explanation:
(Photo courtesy: Getty Images)
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