There were two full moons in January, and now March is
tying the record, with a full moon earlier this week,
March 2, and another due March 31.
Such phenomena are rare, says Joseph Hollweg, University of New Hampshire professor of physics. "On average there are only seven 'blue moons' -- what people call the second full moon in one calendar month -- in 19 years," says Hollweg. "To get two blue moons in a year, that won't happen again until 2018."
The reason for two full moons in one month is the Moon's synodic period, the time required for the Moon to go from one phase to the next -- say, from full moon back to full moon. "The synodic period is 29 and a half days, so in principal you can fit two full moons in one month, except February, with its 28 or 29 days," Hollweg explains. "And so, February can never have a blue moon," he adds. In 1999, however, there was no full moon in February, another rarity.
Why a "blue moon?" Hollweg says the term first arose in the Middle Ages, to refer to someone who argued "the moon is blue, someone who would argue anything, no matter how wrong." By the late 1800s, when the volcano Krakatoa erupted in the Pacific, the Moon did indeed look blue due to volcanic ash trapped in the atmosphere. At that point, the phrase "once in a blue moon" denoted a rare event. Only in the 1940s did "blue moon" come to mean the second full moon in a month.
Is there any scientific benefit to two full moons in a month? "None whatsoever," laughs Hollweg. But what about the chance for us backyard astronomers to peer at two full moons in a month?
"Unfortunately, a full phase is the dullest time to look at the Moon," says Hollweg. With the Sun shining on the full surface, there's little to see because of the lack of shadowed craters or definition. The best bet for Moon watchers? Wait until mid-month, for the crescent and half moons, expected between March 10 and 14, when the Moon will be visible in the morning hours, and between March 19 and 24, when the Moon will be visible in the evening.
Prof. Hollweg is available for further comment at 603-862-3869, or call Carmelle Druchniak, UNH News Bureau, at 862-1462.
By Carmelle Druchniak
UNH News Bureau