Blue Flower Moon rises Saturday

Blue moons, by either definition, are certainly not frequent, but they are not rare.

Posted 1:25 p.m. May 17, 2019 — Updated 1:45 p.m. May 17, 2019

Tony Rice

, WRAL contributor/NASA ambassador

The Algonquin Native Americans called May’s full moon the flower moon. Saturday’s full moon can be called the blue flower moon.

But Saturday is May 18. If a blue moon is the second full moon in a calendar month, and it takes about a month (it's where the word comes from) for the Moon to cycle through its phases, shouldn’t a blue moon occur only on the 30 or 31?

There is a second, lesser used, definition of blue moon -- the third of four full moons in a season.

"The phrase 'Blue Moon' has been around a long time, well over 400 years, and during that time its meaning has shifted," said Philip Hiscock, a folklorist from the Memorial University of Newfoundland. "I have counted six different meanings."

The phrase originally was used to describe something absurd. It then shifted to mean “never."

The seasonal definition was first seen in the 1943 edition of the Maine Farmers Almanac. A 1946 article in Sky and Telescope magazine provided the monthly definition. The phrase fell out of use for 30 years.

Hiscock’s research found a mention in the 1970s radio program “Star Date” using the monthly definition. The Trivial Pursuit board game featured the monthly definition in a question as well. When a newswire service wrote about the full Moons of May 1 and May 31, 1988, that monthly definition was picked up by newspapers, radio stations and television newsrooms.

Blue moons, by either definition, are certainly not frequent, but they are not rare. The month-based definition occurs about every 2.41 years on average, while the seasonal one occurs a little less often, about every 2.68 years on average. According to astrophysicist Neil deGrasse Tyson, blue moons are “more frequent than presidential elections, yet nobody calls them rare.”

The name has very little to do with the color. Expect the same white appearance as it rises higher into the sky and a more yellowish color at moonrise and moonset. All that extra atmosphere the light must pass through increases the effect of rayleigh scattering, which scatters blue light, leaving frequencies more toward yellow and and orange.

Just as the atmosphere can make a moonrise (or sunset) look more yellow, it can make the Moon look more blue, but the conditions required are rare. Ice crystals, sand or even water droplets in the atmosphere can scatter more red and green light, leaving blue, but these conditions are rare. These particles must be similar in size, about a micron in diameter. For comparison, the more commonly seen droplets of water in clouds are about five times larger.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on Twitter @rtphokie.

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