by Katie Bowell, Curator of Cultural Interpretation
For those readers lucky enough to live in the Eastern Hemisphere (Europe, Africa, Australia and Asia), you’ll have a treat on December 31st – the last lunar eclipse of the year. Lunar eclipses occur when the Moon passes through the Earth’s shadow. The more central the moon in the shadow, the more total the eclipse. This eclipse, known as a minor partial eclipse, won’t be too spectacular – just a tiny sliver of the Moon (~7.6%) will go dark as it passes through the Earth’s shadow. However, no matter where you live, we’re all in for another treat this New Year’s Eve, something that only happens once in a blue moon. What’s that? A Blue Moon.
Full Moon from Air & Space Museum
Today, the expression “blue moon” has two meanings: (1) something rare and (2) two full Moons occurring in one month. However, those meanings have changed over time (the last 400-ish years, to be exact).
The earliest recorded use of the phrase “blue moon” happened in 1528, in a pamphlet criticizing the English clergy. The line, “Yf they say the mone is belewe/We must believe that it is true” (If they say the moon is blue, we must believe that it is true), was used to describe an impossible event. By the 18th century, the phrase “until a blue moon” was used to mean “never.” So why, today, do we used the expression “once in a blue moon” to mean something that happens every now and then, or rarely? Well, it turns out the moon can appear blue.
When the Indonesian volcanic island Krakatoa erupted in 1883, the particles of ash it shot into the atmosphere were around 1 micron wide, the right size to scatter red light while letting other colors pass through. Sunsets looked green and the Moon looked blue all around the world for almost two years. Blue Moons could happen. A similar phenomenon happened in 1927 when an extra-long dry season created enough dust in the air for a blue Moon in India, and in 1951, smoke from forest fires in Canada turned the Moon blue over North America.
So when I say that we’ll see a blue Moon this Thursday, am I psychically predicting a forest fire or volcanic eruption? Happily, no. Besides, if I had psychic powers, I’d want to know lotto numbers, not the color of the Moon.
Here’s where the story continues: In North America, the expression “blue moon” also came to mean the third of four full Moons in a season. Traditionally, almanacs (the handy publications that tell you things like when the Moon was going to be full) worked on the tropical year, which goes from Winter Solstice to Winter Solstice, instead of from January 1st to December 31st. Most tropical years have 12 Moons, three per season (winter, spring, summer, and fall). But every once in a while, a tropical year will have 13 Moons – giving one season four Moons instead of three. The third of those four Moons was the one that didn’t belong, and came to be called the “Blue Moon.”
1853 Maine Farmers' Almanac from Old Farmer Almanacs
The switch in definition from “Blue Moon” meaning the third of four full Moons per season to meaning the second full Moon in a calendar month happened in 1946, when a writer for Sky & Telescope Magazine misinterpreted the Maine Farmers’ Almanac’s listing of a “Blue Moon.” The new definition caught on, which is why on Thursday the second full Moon of December will be a “Blue Moon.”
So, why are there two full Moons this month? We usually only have one full Moon per month because there are 29.53 days between each moon. However, get a month that’s long enough (31 days) with a full moon happening early enough (this month the first one happened December 2nd), and you can have two full Moons in one month. The definition of a “Blue Moon” as something rare still holds up, too, since these double Moons only occur once every 2.7 years.
Happy New Year and, as always, “Good seeing.”