Blue moon tonight

[AustinBoston]

Thursday, May 31, 2007, is a blue moon.

What is a blue moon, you ask?

It turns out to depend on who you ask.

The true definition of a blue moon is "The third full moon in a three-calendar-month period that has four full moons."

Unfortunately, about 60 years ago, an incorrect definition was published in "Sky and Telescope" magazine.  The magazine quickly retracted, but the damage was done.  The incorrect definition, which is much more widely known than the original, states that a blue moon is "the second full moon in a calendar month."

The two definitions usually (but not always) come to the same conclusion.

There are typically about 40 blue moons in a century, so they are not common, but every so often, a blue moon can occur in both January and March of the same year.

By the second definition, a blue moon can never occur in February, but such is not the case under the original definition.

So now you know (or are more confused than ever).

Austin