Tonight's full moon is commonly referred to as a blue moon—the second full moon, according to popular belief, to rise in a calendar month. Although folks have been keeping track of the moon's patterns for millennia, that sense traces back less than a century. And back in the early 19th century, when blue moon first appeared in print, its meaning was metaphoric, naming "a very long period of time." Then, as now, blue moon commonly appeared in the phrase once in a blue moon: that is, with the sense "very infrequently." (来源：专业英语学习网站 http://www.EnglishCN.com)

Why blue? Astronomers know that, very rarely, dust particles in the high atmosphere cause the moon to appear blue to those of us on earth. The1883 eruption at Krakatoa left us with a blue moon for almost two years, for example, and so did widespread forest fires in Canada in 1951.

But in fact, two full moons in a given calendar month are far from rare; they occur predictably, if not regularly, at least once every two to three years, depending on how the lunar and the solar cycles mesh. So how did blue moon come to develop its recent metaphoric meaning? Believe it or not, it was by a simple misunderstanding—a misunderstanding that appeared in a magazine article and was repeated on the radio. And while some folks might argue that blue moon "oughtn't" have the meaning it does, there's no doubt that sense is well-established.