A blue moon isn't blue. Nor is "once in a blue moon" so rare, since it happens once every 2 1/2 years.
Nonetheless, blue moons persist, in both song and saying. And tonight's blue moon has the added twist of landing on New Year's Eve -- so don't, as songwriters Rodgers and Hart would have it, let it see you standing alone.
For all its lyrical nature, though, a blue moon is actually just a quirk in the calendar. Any time a month gets two full moons, the second one is called a blue moon.
That happens because it takes about 29 1/2 days to complete a phase of the moon, from new moon to full moon. Since that doesn't coincide with the number of days in a month, sometimes a month will get two full moons. This month, for example, got one on Dec. 2 and will see another tonight.
Additionally, the moon, sun and Earth are currently in a rare alignment that may lead to flooding, government forecasters have said.
No one is quite sure who first started calling the second full moon of a month blue.
"My impression is it's just sort of an expression that cropped up," said Guy Ottewell, a Greenville, S.C.,-based astronomer who compiles an annual astronomical calendar.
Although blue moons don't have their own myths, they share in the usual lore that surrounds all full moons -- that there's more crime, for example, or that birth rates somehow follow the ups and downs of the lunar phase. Studies, however, have tended to disprove those myths.
"People get concerned about them. They think people are more likely to be out of control, to run amok," said Kim Long, author of "The Moon Book." "There may be some indications that that's partly true, but certainly it's been overplayed."
Still, the myths persist, probably because the moon has been a compelling symbol throughout the ages, he said.
"It's just that it's so visible, and its appearance changes so obviously," Mr. Long said. "It's been an obvious magnet for people to create myths and legends about."
But to serious astronomy buffs, full moons aren't particularly intriguing. And, in fact, they get in the way of stargazing.
"You usually try to avoid full moons because they're too bright and they dim the rest of the sky," said Todd Ullery, a producer at the Davis Planetarium at the Maryland Science Center. "And the features don't show up on the moon either because you don't get any shadows."
More interesting than the blue moons that occur because of the calendar are moons that actually are bluish in color. Now, those are truly rare, and in fact, they are probably the subject of the "once in a blue moon" saying.
"If the moon is any color other than white, it tends to be yellowish or reddish," Mr. Ottewell said. "So it's rarer for it to be blue."
The only time the moon will appear bluish is after something like a volcanic eruption or a big forest fire sends particles of a certain size into the atmosphere. Those particles scatter light in the longer wavelengths, such as yellow and red, leaving only the shorter wavelengths of blue and green visible to the naked eye.
These, of course, aren't as predictable as the calendar blue moons. The most famous of really blue moons occurred in 1883 and 1884 after the Krakatoa Volcano erupted.