Once in a Blue Moon

Tonight is a blue moon, which means (according to the modern definition) that the moon is full for the second time in one calendar month.

Blue moons can't occur in lunar calendars, because a new month starts every 28 or 29 days, usually tied either to the new moon (meaning it never rises) or the full moon (everyone knows what that means). But in our calendar, with its months of 30 and 31 days, blue moons can occur from time to time--on average, about every two and a half years. Which means they're rare, but not as rare as Leap Year.

Still, "once in a blue moon" is indeed a more poetic and interesting way of saying something's uncommon than "once every leap year," in part because leap years occur at regular, well-known intervals, while the average lay-person would have no way of saying definitely when the next blue moon would occur--and indeed, this blue moon, which is in May where I live, might occur in June somewhere else.

Tonight's blue moon is in Sagittarius, which is my sun and moon sign, so I'm taking this as a time of great power for me. I might even do something to celebrate.