Celestial News: May’s full moon — blue or not? | SteamboatToday.com

Celestial News: May’s full moon — blue or not?


Jimmy Westlake
For Steamboat Pilot & Today
Our full moon this month falls on May 18. It will be the only full moon in May, but it will be the third full moon in a season containing four. Does this qualify it as a blue moon or not?
Jimmy Westlake/courtesy

Will our full moon on May 18 be a blue moon or not? Well, it depends on what the meaning of the term “blue moon” is.

As the moon orbits the Earth, it waxes and wanes through a cycle of phases that repeats itself each month. Normally, there are 12 full moons in a calendar year, three for each season. These 12 full moons have been given traditional names that can be traced back to early American or Native American folklore.

For example, the season of spring has its Worm Moon, Egg Moon and Strawberry Moon. Occasionally, a troublesome 13th full moon will show up in a calendar year, giving one season an extra full moon. What should this one be called? 

Calendar-makers of long ago used red-colored moon symbols on their calendars to mark the three named full moons of each season, and they used a blue-colored moon symbol for the unnamed full moon. By tradition, the third full moon in a season containing four was denoted as a blue moon. This was to make sure that each season would begin and end with its traditionally named full moons.

At least, that’s one story about the origin of the term. We’ll call this a blue moon of the first kind.

The month of phases, from one full moon to the next, is 29.5 days long, just 12 hours shy of 30 days. This causes the full moon to happen at least half a day earlier each successive month until the date of the full moon creeps its way forward to the beginning of a month.

When a full moon falls on the first or second day of a month, it’s possible to fit a second full moon into a calendar month of 30 or 31 days. Two full moons in one calendar month happens about once every 19 months. February, containing 29 days at the most, can never host two full moons. 

In recent years, the popular meaning of blue moon has changed to denote the second full moon in any calendar month instead of the third full moon in a season containing four. This new definition is a boo-boo that can be traced back to an article in the March 1946 edition of Sky & Telescope magazine, where author James Hugh Pruett apparently misinterpreted the definition of a blue moon from an earlier 1937 Maine Almanac.

The error was perpetuated in January 1980 during the popular radio program “Star Date” and the rest is history. Nowadays, almost everyone thinks of a blue moon as being the second full moon in any calendar month. We’ll call this a blue moon of the second kind.

This year’s Worm Moon fell on March 20, the same day as Spring Equinox. This very early first full moon of spring allows time to fit in a fourth full moon before summer begins. By the original, seasonal definition of the term, the third full moon of spring on May 18 will be a blue moon of the first kind, however, there is still only one full moon in May. By the modern, popular meaning of the term, our full moon this month is not a blue moon of the second kind, but just a nameless full moon of spring. 

So, is our full moon on May 18 a blue moon or not?  It just depends on your preferred definition of the term blue moon. There’s no reason why we can’t embrace both definitions, in which case, the full moon on May 18 will be a blue moon of the first kind.

Our next blue moon of the second kind happens on Halloween, October 31, 2020.

Oooooh — scary!

Jimmy Westlake retired from full-time teaching at Colorado Mountain College Steamboat Sorings in 2017, after 19 years as professor of physical sciences. His Celestial News column appears monthly in the Steamboat Pilot & Today. Check out Westlake’s astrophotography website at jwestlake.com.


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