According to modern folklore, the second full moon in a calendar month is called a "blue moon." As expressed in Pacific time, there is a blue moon on July 31st.
The moon goes through a complete cycle of phases in 29-1/2 days. A month was originally defined as the time it takes the moon to go through one cycle. Each month began with the sighting of the new moon, but for reasons of convenience our civil calendar has been regularized so that months have fixed lengths of 30 or 31 (or even 28 or 29) days. Because the calendar month is slightly longer than the actual time it takes the moon to go through one cycle of phases, each phase must occur once a month. If, however, a certain phase occurs within the first day or two of the month, that same phase can occur again before the month is com-plet-ed, and there can be two full (or two new) moons in the same calendar month. (This is not true for February which occas-ionally has only three of the four lunar phases and which is always too short by at least half a day for any phase to be repeated.)
This year, the moon is full at 4:09 a.m. P.D.T. on July 2nd and again at 11:05 a.m. on July 31st. This second full moon is a "blue moon."
The expression "once in a blue moon," as referring to a rare event, dates to the sixteenth century. Calling the second full moon of a month a blue moon is a much more recent convention, and it dates only to 1946 when an editor of the popular astronomy magazine Sky & Telescope erred in an article on moon phases. He misinterpreted the 13th full moon in a "tropical year" (the year measured from winter solstice to winter solstice) as the second full moon in any month and unwittingly created a new definition. This new definition was popularized in the 1980s and apparently is here to stay.
A blue moon occurs, on the average, once every 2.7 years, but the actual interval varies considerably. The last blue moon (in Pacific time) happened on July 31, 2004.The next will be on December 31, 2009.
A "blue moon" looks no different than any other full moon. The full moon – "blue" or not – rises at sunset and is visible all night long.
For more information on blue moons, see "Folklore of the "Blue Moon" by Philip Hiscock at http://www.GriffithObs.org/IPSBlueMoon.html.
For information on the Observatory's programs, please call (323) 664-1191. The Obser-vatory's World Wide Web site is http://wwwGriffithObs.org.
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Originally Posted by PinHead
"whats the time"
