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(   Tuesday night's full Moon will be a "Blue Moon", meaning the second full Moon in a calendar month, except it will be the first full Moon of August, so it really shouldn't be a "Blue Moon". But it is. Got that?   ( divider line
    More: Misc, Sky & Telescope, full moons, Trivial Pursuit, midsummers  
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2593 clicks; posted to Geek » on 16 Aug 2013 at 4:33 PM (4 years ago)   |   Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»

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2013-08-16 07:02:53 PM  
1 vote:
Well, there are three ways we measure time that are based on our local astronomy.  One "day" marks one complete rotation of the earth around its axis.  This is complicated by the fact that the earth is also revolving around the sun but the concept of noon is pretty clear, that's when the spot you are standing on is at its closest to the sun for the day.  One sidereal "year" marks one rotation of the earth around the sun.  Due to the earth's wobble (or precession) this is slightly different than a tropical or solar year but either is approximately 365.25 days.  A lunar "month" is the based on the sun, earth and moon being as colinear as possible (which is known as syzygy) and that's around 29.5 days.  BTW a week was originally just a division of a lunar month into four equal chunks...

Leap years are an attempt to deal with that extra day problem every four years and intercalary (aka blue) moons are an attempt to deal with those extra months that happen every 2-3 years.  This attempt to sync up systems of time is called Intercalation. Some people propose that "blue moon" really means "betrayer moon" (belewe is apparently an archaic word for betrayer) because it would trick folks into thinking some part of the Christian calendar (like lent and so on) had started or ended earlier than it was really supposed to happen.  Anyhow there is definitely a historical and physical basis for this even though our current calendar months no longer line up with the phases of the moon.
2013-08-16 05:31:27 PM  
1 vote:
There's a good wikipedia entry on Blue Moons.  The incorrect "two full moons in a month" definition was started in the March 1946 issue of Sky and Telescope.  A blue moon has always meant the third full moon in a season with four full moons and the seasons always have precise start and end times at the equinoxes (when a line from the center of the sun to the center of the earth crosses the equator) and solstices (when that line hits the maximum latitude at the tropic of cancer/capricorn.)

One really cool but mostly forgotten phenomenon that relates to this is the Metonic Cycle which is due to the fact that 19 solar years is almost exactly the same time as 235 lunar months.  235 = (12 * 19) + 7 so there are always seven blue moons over a span of 19 years.
2013-08-16 04:43:54 PM  
1 vote:
So... how many new smurfs does this generate?
2013-08-16 04:31:04 PM  
1 vote:
That actually makes WAY more farking sense than "2 blue moons a month."
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