Scientific American:
This Thursday's full moon carries the title of "Harvest Moon" for those living in the Northern Hemisphere. But what gives the special moon its name?The only full moon names I ever remember are the Harvest Moon and the Hunter's Moon.
The moon officially turns full when it reaches the spot in the sky opposite (180 degrees) from the sun. That moment will occur on Thursday (Sept. 19) at 7:13 a.m. EDT (1113 GMT).
Thursday's full moon is the one nearest to the September equinox this year, making it the Harvest Moon by the usual definition. Other definitions of the Harvest Moon, according to Guy Ottewell's Astronomical Calendar 2013, are the full moon on or after the date of the equinox, or the full moon in October. [The Moon Revealed: 10 Surprising Lunar Facts].
Although we associate the Harvest Moon with autumn, this year's version is actually the last full moon of the summer season. In fact, it's this summer's fourth full moon, an oddity of sorts since most of the time there are only three full moons per season.
Since this summer had four, the third full moon is designated as a Blue Moon, which was indeed the case last month. The 2013 Harvest Moon comes less than 3.5 days prior to the Autumnal Equinox, although a Harvest Moon can occur as early as Sept. 8 (as will be the case next year) or as late as Oct. 7 (as was the case in 1987).
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