Once every 2.73 years.
It just doesn’t have the same ring. “Once in a blue moon sounds better,” said Geoff Chester, an astronomer at the National Air and Space Museum’s Albert Einstein Planetarium in Washington, D.C.
Tonight is a blue moon, the second full moon in a month. The phrase “once in a blue moon” probably refers to how seldom blue moons occur, on an average of once every 2.73 years.
But why “blue”?
“Frankly, we don’t know where the term comes from,” Chester said.
It’s certainly not an issue of hue.
“The moon doesn’t turn blue,” said Arnold Pearlstein of the Space Place Planetarium in Miami. “You’re going to see more of a gold moon.”
Dust and pollution in the atmosphere give the moon an orangish or gold tint.
Blue moons have become part of our culture, largely remembered from the popular Rodgers and Hart standard: “Blue Moon, you saw me standing alone, without a dream in my heart, without a love of my own.”
That song was recently revived by the singing mice in the movie “Babe.”
There have been times when the moon turned blue. One of the most prolonged blue moons was in 1883, when a massive volcano erupted in Krakatoa, spewing ash and soot into the atmosphere for weeks.
The other type of blue moons can occur in 11 months of the year. “It can never happen in February,” Chester said. “Not even in a leap year.”
That’s because it takes 29.5 days for the moon to go through its phases, so February is never long enough to squeeze in two full moons.
There’s not much unusual about this blue moon. But you might want to watch how you use the term “once in a blue moon” around the turn of the century, the time of the next blue moon.
In 1999, there’ll be a blue moon in January and March.
It’s an example of how the caprice of a man-made calendar can cause the interval between blue moons to be longer or shorter than the average of 2.73 years.
While there’s not much scientific interest in the blue moon, the full moon offers a chance to experience what Chester calls the “moon illusion.”
When the moon rises it looks enormous. As the moon moves overhead it appears to get smaller.
“Your mind is playing tricks on you,” Chester said. “It relates the size of the moon to the size of objects you know, like houses.”
As the moon moves away from familiar objects, the mind stops the trick.
To see how the illusion works, Chester recommends looking at the rising moon - it should come up in the east, just after sunset - through a paper towel tube. Then look through the tube when the moon is overhead. Because the tube blocks out other objects, the moon will appear to be the same size in both spots.
You may want to avoid this experiment if you enjoy looking at a big rising moon hanging over the ocean.
“My boss did (it), and he can’t get the illusion to come back,” Chester said.