Update for Aug. 1: The Blue Moon of July 31 wowed skywatchers around the world. See photos and our full story, read: Blue Moon of 2015 Thrills Skywatchers with Lunar Beauty: Photos 
 
 On Friday, much of the world will have the opportunity to observe a Blue Moon: A somewhat rare occurrence that doesn't have anything to do with the moon's color. 
 
 During most years, the Earth experiences 12 full moons, one in each month. But some years, such as 2015, have 13 full moons, and one of those "extra" lunar displays gets the label of Blue Moon. 
 
 The lunar or synodic month (full moon to full moon) averages 29.530589 days, which is shorter than every calendar month in the year except for February. Those extra one-half or one-and-one-half days accumulate over the year, causing some years to have 13 full moons rather than 12. [Video: What's a Blue Moon, Is It REALLY Blue?] 
 
 Thought to be called "blue" after an old english term meaning "betrayer," a Blue Moon is an extra full moon that occurs due to a quirk of the calendar. [ See the full Blue Moon Infographic here .] Credit: Karl Tate, SPACE.com 
 
 To see what I mean, here is a list of full-moon dates in 2015: Jan. 5, Feb. 3, March 5, April 4, May 4, June 2, July 2, July 31, Aug. 29, Sept. 28, Oct. 27, Nov. 25 and Dec. 25. In 2016, the first full moon falls on Jan. 23, and each calendar month has only one full moon. 
 
 The expression "once in a blue moon" has a long history of being used to describe rare events; but it was also used in the Maine Farmers' Almanac to describe the third full moon in a season that has four (normally, a three-month season will only have three full moons). 
 
 In 1946, Sky & Telescope magazine published an article that misinterpreted the older definition, defining a Blue Moon as the second full moon in a calendar month. This has become the most recent and perhaps most widely accepted definition of a Blue Moon. And hence, the full moon on July 31 is referred to as a Blue Moon, because it was preceded by the full moon on July 2. By this definition, a Blue Moon occurs roughly once every 2.7 years. 
 
 The full moon appears to last for at least the length of one night, but technically speaking, it is an instantaneous event: It occurs when the sun, Earth and moon fall close to a straight line. It takes place at the same instant everywhere in the world, whether the moon is above or below the horizon. 
 
 
 
 Supermoon! Not only is it the biggest full moon of the year, it's super cool. 
 
 Full Moons: They come around every month and light up the night. 
 
 Lunar Eclipses: There's nothing more spectacular than a blood-red moon in Earth's shadow. 
 
 Blue Moon: They're rare and amazing, if not actually blue. 
 
 Solar Eclipses: The moon has the power to block out the sun! 
 
 
 
 The full moon on July 31 occurs at exactly 6:43 a.m. EDT (1043 GMT). 
 
 So, when you look at the Blue Moon on Friday morning, don't expect to see a different color scheme (although it is possible for the moon to appear to have a bluish hue). Just be aware that the so-called Blue Moon is a byproduct of the contrast between the calendar month and the lunar month. 
 
 Editor's note: If you capture an amazing view of the Blue Moon full moon of July 31 and would like to share it with Space.com for a story or gallery, send images and comments in to managing editor Tariq Malik at spacephotos@space.com. 
 
 This article was provided to SPACE.com by Simulation Curriculum, the leader in space science curriculum solutions and the makers of Starry Night and SkySafari. Follow Starry Night on Twitter @StarryNightEdu. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com. ||||| 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]
– When Friday's blue moon arrives, don't expect it to be blue—a blue moon isn't actually that color, reports CNN, though some full moons can indeed have a bluish hue. The phrase "once in a blue moon" refers to something that is rare, and it was once used this way in the Maine Farmers' Almanac when describing the third full moon in the rare season that has four (typically, there is only one full moon a month, thus a three-month season will have three full moons). But in 1946, Sky & Telescope magazine published an article that misunderstood this definition, instead calling the second full moon in a calendar month a blue moon. And it is indeed fairly rare: It happens once every 2.7 years or so. This has become the modern definition and is used to describe Friday's full moon, which is the second this month (the first was on July 2); it'll be the last of its kind until January 2018. Interestingly, while the most recent blue moon according to this modern definition occurred at the end of August of 2012, the most recent blue moon according to the original definition occurred more recently, in August of 2013, when the full moon was the third of four that summer. As for the blue moon on July 31, while it may seem to last all night, it's technically an "instantaneous event" that occurs at 6:43am EDT on the nose, reports Space.com. (Earlier this month, a man took an incredible picture of the International Space Station passing in front of the moon.)