Q. We will have what older people call a "blue moon" on Dec. 31. Can you tell me how often this occurs? When was the last one? -- C.S.
A. These days, a blue moon is usually considered to be the second full moon in one calendar month. We will have a full moon on Dec. 2, and another -- the blue moon -- on Dec. 31. The last time this happened was in 2007, when there were full moons on June 1 and 30.
On average, there is a "blue moon" by this definition every 2½ years -- but there is not a specific amount of time between them. For example, 1999 had two blue moons, in January and March.
This definition of blue moon is recent, dating back to a 1946 article in Sky & Telescope magazine. It was popularized by a 1980 broadcast of the National Public Radio program Star Date.
The 1946 article contained a misunderstanding of the blue moons noted by the Maine Farmers' Almanac. An article in the May 1999 issue of Sky & Telescope set the record straight by showing that as far back as 1819, the almanac calculated blue moons not by month but by season, with a variety of "subtle twists" to the calculations.
Simply put, most seasons have three full moons, but occasionally there are four. The almanac used blue moon to refer to the third full moon of a season that had four full moons.
More information about blue moons, including a blue-moon calculator and links to the Sky & Telescope articles, is available at www.obliquity.com/astro/bluemoon.html.
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