Info forwarded by Michael from one of the sites he follows on the "geoblogosphere."
Blue Moon 31 Dec. 09
Thursday, 31. December 2009, 07:26:38Once in a blue moon - is not that far away, it will in fact happen tonight on this very New Year's Eve (2009).
According to popular usage, a blue moon refers to the second full moon that occurs in any calendar month. On average, there will be 41 months that have two full moons in every century. By that calculation 'once in a blue moon' means once every two-and-a-half years - well on average about every 2.7154 years, if you really want to know. A rare event.
The rarest of all blue moons is, however, when it happens on New Year's Eve. Last time was in 1990 and next time will be in 2028. It happens only about once in every twenty years. So do watch out tonight - if the sky is clear, if your head is clear, and there are not too many fireworks to disturb your view. Enjoy a magical moment when magical things happen. It will unfortunately not be visible (before midnight, and thus still in December) in North and South America, I have heard, but will be visible over most of Europe, Africa, and Asia.