Ever hear that expression, "Once in a blue moon"?
We had one of those blue moons back on New Years Eve. In some parts of the world, they had a partial lunar eclipse at the same time.
So just what is a blue moon?
Well, most years have twelve full moons, with one occurring each month. However, in addition to those twelve full lunar cycles, each solar calendar year has roughly eleven extra days compared to the lunar year.
These extra days accumulate from year to year, so about every 2-1/2 to 3 years, there is an extra full moon. And that extra full moon is called a "blue moon."
WHY it's called a blue moon is open to debate. Apparently the expression originally came from the extremely rare, actual visually blue-tinged Moon caused by atmospheric layers of forest fire smoke or volcanic dust at just the right height and visual angle.
I guess since an extra full moon is relatively uncommon, it got to being associated with the really rare visual "blue moon".
And that's why when someone says something happens "once in a blue moon" they are talking about an uncommon event.
There won't be two full moons in one month again until August of 2012, so it's going to be a while.
After all, this only happens once in a blue moon!