"Once in a blue moon
There's a thing called happiness
It happens when you're in
A state of natural grace"

​

~ "Once in a Blue Moon", Van Morrison (2003)

 

 

"Once in a Blue Moon" is an expression used to describe something that doesn't happen very often.

 

Astronomically, a "blue moon" is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar, depending on the astronomical or the current common definition.  

 

Although the term doesn't refer to the actual color of the moon, there was a time, not long ago, when people saw actual bluish-colored moons almost every night. Full moons, half moons, crescent moons - they were all blue, except some nights when they were green!

​

The time was 1883, the year an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb.  Plumes of ash rose to the very top of earth's atmosphere.  And the moon turned blue.

​

Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide - the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

​

Blue moons persisted for years after the eruption. People also saw lavender suns and, for the first time, noctilucent clouds (night shining clouds).

​

Other less potent volcanos have turned the moon blue, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichon volcano in Mexico. And there are reports of blue moons caused by Mt. St. Helens eruption in 1980 and Mount Pinatubo in 1991.

​

By designer Carol Martin, this tartan was inspired by a blue moon in December 2009.  She states: "Where I live in northern Canada, winter nights are dark, but the full moon produces easily-seen shadows on the snow behind objects such as trees. I have attempted to show this effect here."

​

For more on the appearance of actual blue-coloured moons, click the moon photograph above.