Once in a Blue Moon

An artificially colored "Blue Moon" of November 2010 imaged from Brooklyn, New York.

An artificially colored “Blue Moon” of November 2010 imaged from Brooklyn, New York.

The second Full Moon of July 2015 arrives on July 31 at 10:43 Universal Time. No doubt the headline writers of the interwebs will be all over this bit of non-news, but as most readers of Cosmic Pursuits understand, a Blue Moon is simply the second Full Moon of a calendar month. Or in another definition, which does not apply here, a Blue Moon is the third full Moon of a season in which there are four Full Moons. In either case, there’s no astronomical significance to the event, and the Moon will not have a blue tinge. Though any Full Moon in summer makes for a pleasant evening walk when you can smell the thick air and scent of summer flowers in this all-too-brief season.

Two Full Moons in a calendar month is not particularly rare. The last was on August 31, 2012. The next two will occur on January 31, 2018 and March 31, 2018 for some parts of the world. But a truly blue-colored Moon is a far more rare event. Such coloring is caused by scattering of moonlight by fine particles in the Earth’s upper atmosphere. Large forest fires or volcanic eruptions can eject enough powdery material to cause blue-colored Moons. The gargantuan eruption of the Krakatoa volcano in Indonesia in 1883 produced blue Moons visible from much of the world for nearly two years. Some observers reported blue Moons after the eruptions of Mount Pinatubo in 1991, El Chichon in 1983, and Mount St. Helens in 1980. And a massive forest fire in Canada in 1950 resulted in blue and lavender-colored Moons for several days.

With any full Moon, most serious stargazing comes to a halt because of badly brightened skies. So put your telescope away and wander out to see the Blue Moon of July 31, 2015. Then catch up on your sleep because there’s a lot more to see in the night sky this summer.

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