Blue Moon 2013: If Tonight's Full Moon Is August's First, Why Is It Still A 'Blue Moon'?

By Josh Lieberman on August 20, 2013 5:08 PM EDT

full moon
A blue moon rising tonight is the first full moon of August, yet it is still considered "blue." (Photo: Reuters)

A blue moon will illuminate the night sky tonight. Pop quiz: what is a blue moon? If you said "a moon that is blue," you'd be pretty wrong, but also sort of right. If you said "the second full moon in a calendar month," with a self-satisfied grin, you'd be sort of right, but should also probably be taken down a notch. If you said "a pretty decent wheat beer," you'd be technically right, but would be off topic.

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It turns out that the scientific definition of a blue moon is a bit confusing. So here's the deal.

Though tonight's full moon is the first of August, it is actually a blue moon. That's because the original definition of a blue moon is the third of four full moons in a season, and that is still the most valid definition of a blue moon.

A little light math is necessary here. The lunation, or lunar cycle, is 29.53 days; one solar year is 365.25 days. This means there are 12.37 lunar cycles every solar year -- a number slightly larger than cycle of 12 months that makes up our calendar year. That means that 12 calendar months (or one year) are about 11 days longer than 12 lunar cycles. These "extra" 11 days accumulate, so that every two to three years there end up being an additional full moon in a season.

A new definition of a blue moon emerged in 1946, and all because of a mistake in an astronomy magazine Sky & Telescope. In the article "Once in a Blue Moon," amateur astronomer James Hugh Pruett quoted the 1937 Maine Farmer's Almanac (which is unrelated to the more famous Farmer's Almanac), saying that the moon sometimes "comes full thirteen times in a year."

"Seven times in 19 years there were -- and still are -- 13 full moons in a year," Pruett extrapolated from that quote. "This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon."

That's the definition, "two full moons in one calendar month," that has stuck for most people. (It's also a far less complicated definition to understand, so maybe that helped too.)

Then there is the other literal definition of a blue moon, in which the moon appears to be, well, blue. That can occur when particles in the atmosphere cause the moon to take on a hue.

For instance, when Krakatoa erupted in 1883, the moon appeared to be blue for almost two years.

"There was a time, not long ago, when people saw blue moons almost every night. Full moons, half moons, crescent moons--they were all blue, except some nights when they were green," wrote Tony Phillips on NASA's Solar System Exploration site. Large ash particles from Krakatoa "[scatted] red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue and sometimes green."

Similar "blue moons" occurred in 1983 in Chiapas, Mexico, after the eruption of the El Chichon volcano, and in Mount Pinatubo in the Philippines in 1991.

Now that the confusion is cleared up, go enjoy your blue moon tonight. The next one won't be until July 31, 2015, if you go by the "mistake" definition, or on May 21, 2016, if you go by the original definition.

READ MORE:

National Park On The Moon: Lunar Act Would Protect Artifacts In Space

Pluto Moons Named Styx And Kerberos, Despite Overwhelming Vote for 'Vulcan'

Supermoon Live Stream: Where To Watch 2013's Brightest Full Moon [VIDEO]

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