‘Blue moon’ meaning has changed over years

  • Thu Aug 30th, 2012 11:26am
  • Life

You’ve heard the expression “once in a blue moon,” and, guess what, it happens this week.

It’s the second full moon we’ve had this month. The last time we had two full moons in one month was in December 2009, and it happened on New Year’s Eve.

There won’t be another New Year’s blue moon until 2028. After Friday our next blue moon won’t be until July 2015.

According to modern culture a blue moon is considered the second full moon in a given calendar month. This definition has evolved within the past century. Up until about the middle of the past century a full moon was considered the third full moon of any given season that had four full moons.

In any given year there are of course four seasons: winter, spring, summer and fall, and there are usually just three full moons in a season.

That’s because the interval between two full moons, something astronomers call a synodic month is 29.5 days. Since the average month has 30 days you can see why the odds are against it, even if the month has 31 days.

Because of an honest mistake in Sky and Telescope Magazine in 1946, the blue moon definition was pretty much redefined as the second full moon in a given calendar month and ever since then that’s how the blue moon has been defined.

While it may not be exactly true to the old definition, in practice the vast majority of blue moons as they’re defined now would be in season that had four full moons.

Even back then the media had the power to change beliefs. According to the modern definition on the average any given century will experience 27 blue moons. We’re on our fifth one of this 21 century.

Very rarely, the moon will actually sport a bluish tinge. About the only way that can happen is when a volcanic eruption or a forest fire causes atmospheric conditions to make the moon look blue, and that phenomena is even rare in those conditions.

It did happen in many sections of eastern North America in September 1950, due to smoke from widespread forest fires in western Canada. It also happened after the massive eruption of Mount Pinatubo in the Philippines in June 1991, when there were many sightings of a physically blue moon all around the world.

The term blue moon has been in use for hundreds of years. It even shows up in some of Shakespeare’s writings in the 16th century. But why the term blue moon? The truth is that no one really knows. Back then it had more of a frightening tone to it and referred to those very rare times I’ve already described when the moon actually did turn blue.

Many believed it was a bad omen indicating global calamities. A blue moon has also been thought of as a symbol of sadness and loneliness.

Even music reflected this in songs like “Blue Moon” written in 1934 by Richard Rodgers and Lorenz Hart and recorded by tons of artists.

Blue moon or not stargazing will be hampered. Darker and longer nights are coming along late next week once we totally get the bright moon out of the night sky.

Mike Lynch is an astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of the book, “Washington Starwatch,” available at bookstores. Check his website, www.lynchandthestars.com.

The Everett Astronomical Society: www.everettastro.org/.