According to Google, once in a blue moon is about .17 × 10^-8 hertz. And then some other people say that means once every 32 months or so. I’m assuming they’ve done some calculations.
Popular definition seems to agree that a blue moon is the second full moon to occur in one month.
You can also take it for fact that the average interval between full moons is 29.5 days, and the average length of a month is about 30.5 days. This means that it’s not very likely for there to be two full moons, or a blue moon, in an given month. That probability calculates out to give us the numbers showing us that a Blue Moon actually takes place every two and a half years.
Conveniently enough, there is a “Blue Moon Calculator” which lists all sorts of blue moons taking place all over time. The next one is going to be on December 31st, 2009, at 7:11pm.
Fancy that.
Disclaimer: Blue moon meant something a little bit different to farmers and such in the 19th century.
sweet! the next blue moon is on my birthday! i love my birthday. cool things always happen on my birthday.