Blue Moon (2020)
"Blue moon you saw me standing alone / Without a dream in my heart / Without a love of my own" ~ Blue Moon, Richard Rodgers & Lorenz Hart, 1934
There are two types of blue moons commonly reckoned today, either the third of four full moons in a season or a second full moon in a month of the common calendar. Today's moon is of the first traditional type. In North America, the May full moon is known as the Flower Moon, Milk Moon or Planting Moon while in Europe and Britain, it was referred to as the Bright, Grass, or Hare Moon. Although the saying "once in a blue moon," (a phrase which 400 years ago meant something absurd) changed its meaning over time to indicate a rare or improbably occurrence, it never referred to the actual colour of the moon which normally stays on the warmer end of the colour spectrum. However, the moon has appeared bluish, as it did in 1883 after the volcano Krakatoa erupted. Dust in the air acted as a filter, causing sunsets and the moon to turn green and blue all over the world.
"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 A.L. 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.