As Rare as a Blue Moon

Not just a tired old saying, this moon keeps it together

Thursday, May 31, marks the second full moon of the month. Common lore tells us that in any month with two full moons, the second is a Blue Moon.

Every so often, — roughly 42 months every 100 years — two full moons fall within the same month. That’s three and a half percent. Uncommon? Perhaps. Rare? Hmmm.

But some detective work by the writers at Sky and Telescope dug deeper, finding that a true Blue Moon is indeed rare. They followed up on an article by Philip Hiscock, called Once in a Blue Moon, which traced the use of the phrase to the 1937 Maine Farmers’ Almanac.

The writers searched archives and libraries across the country, finding copies of the Maine Farmers’ Almanac dating back to 1819. Among frequent references to Blue Moons, not one was so-called for having two full moons in a month.

The correlation they did find, however, tied the Blue Moon to the four seasons. In any three-month season with four full moons, the almanac dubbed the last a Blue Moon. Typically, a month with multiple full moons comes at the expense of another month’s full moon, thus evening out over a 12-month cycle. However, far less often, the multiple moons result in a year with 13 full moons.

It turns out that much like Leap Year, the Blue Moon is a tool to keep our calendars straight.

April 1 commenced the spring season with the Seed Moon. May 1 marked the Flower Moon and May 31 ends the season with the Strawberry Moon. June 30, rather than taking the name of July’s Blood Moon — is our Blue Moon, which ensures that the names of the full moons remain in sync with the appropriate seasons of the year.

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Illustration: © Copyright 1925 M.C. Escher/Cordon Art-Baarn-Holland; Graphics: © Copyright 2007 Pacific Publishers. Reprinted by permission from the Tidelog graphic almanac. Bound copies of the annual Tidelog for Chesapeake Bay are $14.95 ppd. from Pacific Publishers, Box 480, Bolinas, CA 94924. Phone 415-868-2909. Weather affects tides. This information is believed to be reliable but no guarantee of accuracy is made by Bay Weekly or Pacific Publishers. The actual layout of Tidelog differs from that used in Bay Weekly. Tidelog graphics are repositioned to reflect Bay Weekly’s distribution cycle.Tides are based on National Oceanic and Atmospheric Administration and are positioned to coincide with high and low tides of Tidelog.

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As Rare as a Blue Moon

Not just a tired old saying, this moon keeps it together