A “Blue Moon” Heralds the Harvest

Alan MacRobert, Senior Editor
  617-864-7360 x2151, macrobert@SkyandTelescope.com
J. Kelly Beatty, Senior Contributing Editor
  617-416-9991, jkellybeatty@comcast.net

Note to Editors/Producers: This release is accompanied by publication-quality illustrations; see details below.

Heralding the change in season, a full Moon will rise in the eastern twilight sky on August 31st for the second time this month (the first time came on August 1st). Many people use the expression "once in a blue Moon" to mean something that occurs rarely, and you might be tempted to call August 31st's big, bright orb a "Blue Moon" too. While the former meaning can be traced back centuries, the latter definition is much newer — and it's wrong! At least if you're a stickler about these things.

"In modern usage, the second full Moon in a month has come to be called a 'Blue Moon.' But it's not!" says Kelly Beatty, Senior Contributing Editor for Sky & Telescope magazine. "This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946, and it spread to the world from there."

Sky & Telescope admitted to its "Blue Moon blooper" in its May 1999 issue. Canadian folklorist Philip Hiscock and Texas astronomer Donald W. Olson had helped the magazine's editors figure out how the mistake was made, and how the two-full-Moons-in-a-month meaning spread into the English language.

Before 1946, a Blue Moon always meant something else. For example, says Hiscock, sometimes it referred to an obvious absurdity. Quite a few old songs use it as a symbol of sadness and loneliness. There's even a cocktail called a Blue Moon; it's a mix of curaçao, gin, and perhaps a twist of lemon. And, exceedingly rarely, the Moon actually does turn blue in our sky — when a volcanic eruption, forest fires or dust storms send lots of fine dust into the atmosphere.

Our 1946 writer, amateur astronomer James Hugh Pruett (1886–1955), made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac — which consistently used "Blue Moon" to mean the third full Moon in a season containing four of them (rather than the usual three).

By this definition, there is no Blue Moon in August 2012; instead, the last one was in November 2010, and the next happens in August 2013.

But there's no turning back now. The concept of a Blue Moon as the second full Moon in a month with two, as well as the third full Moon in a season with four, are now both listed as official definitions in the 4th edition of the American Heritage Dictionary (Houghton Mifflin, 2000).

By either definition, Blue Moons happen about once every 2.7 years on average. The last occurrence of two full Moons in a calendar month was on New Years' Eve in 2009. After August 31st, it won't happen again until July 2015.

If you want to tell your readers, listeners, or viewers that this Thursday's full Moon is a Blue Moon, go right ahead. Pretty much everyone else will too. The newer, "wrong" definition is simpler and handier for most people to grasp and use. "That's how the English language shifts. You can't beat back the tide," quips Sky & Telescope Senior Editor Alan MacRobert. "Not when the Moon is pulling the tide."

Sky & Telescope is making two publication-quality illustrations available to our colleagues in the news media. Permission is granted for one-time, nonexclusive use in print and broadcast media, as long as appropriate credit (as noted in the caption) is included. Web publication must include a link to SkyandTelescope.com.

When the Moon is full, it is opposite the Sun in the sky. Thus it rises at sunset and sets at sunrise. Its craters, mountains, and other surface features appear muted because the high Sun casts no shadows as seen from our earthbound perspective. Click on the image for a larger version.
Courtesy Gary Seronik.


When is the Moon "blue," in a calendrical sense? According to the Maine almanac, a Blue Moon occurs when a season has four full Moons, rather than the usual three. This type of Blue Moon is found only in February, May, August, and November, one month before the next equinox or solstice. According to modern folklore, a Blue Moon is the second full Moon in a calendar month. This type of Blue Moon can occur in any month but February, which is always shorter than the time between successive full Moons. Click on the chart for a larger view.
Sky & Telesope diagram

Sky & Telescope, published by New Track Media, has been the world’s leading, most authoritative popular astronomy magazine since its founding in 1941. Sky & Telescope Media also publishes two annuals (Beautiful Universe and SkyWatch), as well as books, star atlases, posters, planet globes, and other fine astronomy products for amateur telescope users and the general public.