How rare is a blue moon eclipse?

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Offline Eric A. Taylor

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How rare is a blue moon eclipse?
« on: 30/12/2009 08:42:41 »
I've just read that there will be a lunar eclipse on December 31, 2009. The full moon on this night will be the second in December marking it as a "blue moon". As if it's not rare enough to have a blue moon on New Years Eve our friends in the East will see a lunar eclipse. I wonder how rare a blue moon eclipse is?

Or does this not count because the eclipse in the East will be on January 1,  2010?

I'd still like to know how rare it is.
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Offline Soul Surfer

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How rare is a blue moon eclipse?
« Reply #1 on: 31/12/2009 14:30:04 »
I'm not sure but it is possible to do some quick guesstimating.  The period of a lunation is about 28 days and months are on average between 30 and 31 days say 30.5   so  2.5/30.5 =  .08  months  say 1 in 12 are blue moons.
so it is about 1 in 12 lunar eclipses are "blue"  there are on average 2 lunar eclipses in a year so we should get a blue lunar eclipse about once every six years or so.  However there will be some periodic factors in this that will make the intermittency variable.
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