Blue Moon Rises, but It Isn't Blue; Princeton Enjoys Two Moons in May

Linda Arntzenius

At least a dozen times a year, there is a chance to walk along the towpath of the Delaware & Raritan canal by the light of a full moon — that is if the weather cooperates with a clear sky.

Rarer is the chance to walk by the light of a full Blue Moon.

Last Thursday, May 31, the Delaware and Raritan Canal Watch issued an invitation to observe the rise of the full moon, a Blue Moon no less, over the towpath on a four-mile walk from the canal bridge at Port Mercer to the Route 1 pedestrian bridge and back. The blue moon, the first in North America since July 2004, was due to appear at 9:04 p.m.

While the weather didn't cooperate enough to entice walkers to the towpath, the moon made its appearance right on cue, before disappearing behind the clouds. It was the second full moon of the month of May, the first being on May 2. And by some definitions, that makes it a Blue Moon.

As seen from the Princeton Battlefield, the moon was, as it happened, not blue but red.

"The moon may appear a bit red if near the horizon and its light passes through thicker Earth atmosphere," explained John Miller, director of the Amateur Astronomical Association of Princeton.

True to the phrase denoting rarity, "once in a blue moon," a blue Blue Moon is rarer than a red Blue Moon, according to moon gazer and science writer Tony Phillips writing in the online magazine Science@NASA.

"Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes. Most are smaller than 1 micron, and they tend to scatter blue light. This kind of cloud makes the moon turn red; indeed, red Blue Moons are far more common than blue Blue Moons."

Nevertheless, on rare occasions, the moon — full or otherwise — has appeared to be blue, as happened after forest fires in Sweden in 1950, Canada in 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.

According to Mr. Phillips, volcanos have turned the moon blue at other times, too. In 1983, after the eruption of the El Chichon volcano in Mexico; in 1980 after the eruption of Mt. St. Helens; and in 1991 following the eruption of Mount Pinatubo.

A Blue Moon is not defined by its color, but rather by its occurrence in the calendar year, about which there is some controversy.

Is it an extra full moon in a quarter year that normally has only three, according to one definition? Or is it the second moon in a calendar month as suggested (some say mistakenly) by the Maine Farmer's Almanac in the 1940s?

"The modern definition sprang up in the 1940s," said Mr. Phillips. "In those days the Maine Farmer's Almanac offered a definition of Blue Moon so convoluted even professional astronomers struggled to understand it. It involved factors such as ecclesiastical dates of Easter and Lent, tropical years, and the timing of seasons according to the dynamical mean sun. Aiming to explain blue moons to the layman, Sky & Telescope published an article in 1946 entitled 'Once in a Blue Moon.' The author James Hugh Pruett (1886-1955) cited the 1937 Maine almanac and opined that the 'second [full moon] in a month, so I interpret it, is called Blue Moon.'

It is usual for months to have only one full moon. But since full moons are separated by 29 days, and most months are 30 or 31 days long, two full moons sometimes occur in a single month. "This happens every two and a half years, on average," said Mr. Phillips, who has a Ph.D. in astronomy from Cornell University,

One thing is clear, however, the phrase "once in a blue moon," which goes back some 400 years, pre-dates the astronomical naming to which it is more likely to have given rise than the other way around.

To complicate matters further, there are time zones. May 31 in one time zone is already June 1 in another. So which month is entitled to the extra moon, May or June? The Blue Moon observed over Princeton last Thursday, would not be a Blue Moon for observers in London, for example, who would not see a Blue Moon there until June 30.

The definition of a Blue Moon as the second full moon in a calendar month, while it may be a bit of modern folklore, has stuck.

Last Thursday, the moon retained her mystery. Moon gazers will have to wait until November 2009 for the next Blue Moon sighting, over Princeton that is. Those in London will have to wait until the end of December of that year.

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