A New Year's Eve like this one comes around once in a blue moon. Look up at the sky Dec. 31 and see for yourself.
For the first time since 1990, revelers will be able to ring in the new year under the light of a blue moon, popularly understood as the second full moon in a month. Just don't expect the light itself to be blue.
"It's not at all clear where the term blue comes from," said Dale Smith professor of physics and astronomy and planetarium director at Bowling Green State University. "The moon is virtually never blue."
Virtually, but not never.
Lawrence Anderson-Huang, professor of physics and astronomy at the University of Toledo, explained, "There are occasions when scattering in the Earth's atmosphere can make the moon look particularly bluish.
The key to this blue coloring is the presence of large numbers of particles in the air that are slightly wider than the wavelength of red light with no other sizes present, according to NASA. When that happens, as with some volcanic eruptions or forest fires, red light is scattered and the white moonlight shining through appears blue. The most notable example of this was in 1883 when the Indonesian volcano Krakatoa exploded, spewing ash into the atmosphere and giving the moon a blue appearance for years.
Those types of blue moons can't be predicted, but the ones measured purely by the calendar come like clockwork. There are 29 days between full moons, so while most months only have one, every two or three years there are bound to be two full moons in a single month even though it's a tight fit, BGSU's Mr. Smith said. This month features full moons on Dec. 2 and Dec. 31.
February is the only month that can never have a blue moon by this definition, which goes back to a 1946 article published in Sky & Telescope. The thing is, that author apparently misinterpreted a Maine almanac's older system in which a blue moon was the third moon in a season of four full moons. Toledo native Donald Olson called attention to the mistake in 1999, but the more modern concept of blue moons remains the accepted one.
Today the phrase "once in a blue moon" suggests something that happens very rarely, but back when the connection between the moon and the color blue first made it into print - all the way back in 1528 - it meant something that never happened at all, according to Morris Dictionary of Word and Phrase Origins.
Who knows exactly how the meaning changed with the years or precisely why the color became associated with the moon in the first place. What can be said with more assurance is that blue moons today occupy a more prominent standing in popular culture than astronomy, where it's seen as more of a calendrical quirk.
"It doesn't have any real significance scientifically," said Dave Weinrich, president-elect for the International Planetarium Society.
Still, he's glad the colorful phrase captures the public's imagination, and not just by inspiring popular, poetic songs but by getting people interested in the heavens - at least for one night every couple of years.
"Anytime you can get people out to look at the real sky to me is a great plus," he said.
Enjoy it while you can this New Year's Eve. That date won't see another blue moon until 2028.