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] ||||| Updated for 9/30: Friday's sky (Sept. 30) is host to a somewhat unusual lunar event in the Western Hemisphere: a second new moon in a single month, which some people call a "Black Moon." 
 
 While a full moon refers to the moment when the moon's Earth-facing side is fully illuminated by sunlight, a new moon refers to the moment when the moon's Earth-facing side is fully in shadow. (Unfortunately, that means the Black Moon will be more or less invisible, even if the moon is high in the sky). 
 
 The lunar calendar almost lines up with Earth's calendar year, so there is typically one full moon and one new moon each month. A second full moon in a single calendar month is sometimes called a "Blue Moon." A Black Moon is supposedly the flip side of a Blue Moon: the second new moon in a single calendar month. The next Black Moon takes place on Sept. 30 (in the Western Hemisphere). [The Moon: 10 Surprising Facts] 
 
 With a pair of binoculars or a small telescope, many spectacular features can be spotted on the moon. See how to observe the moon in this SPACE.com infographic Credit: Karl Tate, SPACE.com 
 
 From the Western Hemisphere, the new moon occurring on Friday, Sept. 30, is a Black Moon. Officially, it occurs at 8:11 p.m. Eastern Time (5:11 p.m. Pacific Time). 
 
 For the Eastern Hemisphere (Europe, Africa, Asia, Australia), this new moon occurs after midnight on the calendar date of Oct. 1. So for this part of the world, this particular new moon is not the second one in the calendar month, but rather, the first! So it does not qualify as a Black Moon, and that hemisphere will have to wait until the end of the month for theirs. Indeed, for the billions living in the Eastern Hemisphere, the Black Moon will arrive on Oct. 30 or, if you live in eastern Asia, Japan, Australia or New Zealand, not until Oct. 31 (Halloween). 
 
 The Black Moon is a somewhat unusual celestial event — they occur about once every 32 months. 
 
 Slooh Community Observatory will host a webcast Monday, Oct. 3 at 3 p.m. EDT (1900 GMT) to show live views of the young moon, viewable on their website and also on Space.com. The webcast will feature discussion with special guests including Slooh's spiritual correspondent Helen Avery, who will discuss the Black Moon's significance to pagan religion, and there will also be a discussion of how to define the Black Moon when dealing with multiple time zones. 
 
 At its "new moon" phase, the moon is always black. It happens at that time of the month when the moon passes through the same part of the sky as the sun and as such, the moon's dark or unilluminated side faces Earth. So there really is nothing to see. 
 
 Actually, that's not always true, since there are times when the new moon passes directly between Earth and the sun and Earthlings can then see the moon's black silhouette crossing in front of the sun, causing a solar eclipse. That, in fact, actually happened with this month's first new moon, on Sept. 1, creating an annular eclipse (also known as a "Ring of Fire Eclipse") over parts of Africa. 
 
 If you have ever wondered where the term "new moon" originated, it simply refers to the start of a new lunar cycle. The time frame from one new moon to the next is called a synodic month, which, on average, lasts 29.53 days. This is the period of the moon's phases, because the moon's appearance depends on the position of the moon with respect to the sun as seen from the Earth. The word "synodic" is derived from the Greek word sunodikos, which means "meeting," for at new moon, the moon "meets" the sun. 
 
 But unlike a "supernoon" which gets countless numbers of people scurrying for vantage points to see a slightly larger and slightly brighter-than-average full moon, with a Black Moon, you simply can't see it. In my opinion, this is the chief reason why Black Moon is going to have a tough time in becoming a popular media brand. 
 
 A couple of evenings later, however, on Oct. 2, you'll be able to pick out a slender sliver of a waxing crescent moon low in the western twilight sky about 30 or 40 minutes after sunset local time. That will also mark the start of Rosh Hashanah, the Jewish New Year 5777. And the following day (Oct. 3) is the Islamic New Year 1438, on the first day of Muharram, the first month in the lunar Islamic calendar. 
 
 Some people mistakenly refer to the appearance of any thin lunar crescent as the "new moon." This fallacy has even spread into popular literature. In his classic work "A Night to Remember," about the sinking of the Titanic, author Walter Lord quotes a fireman in a lifeboat who caught sight of a narrow crescent low in the dawn sky and exclaimed, "A new moon!" 
 
 As one who has been involved in the broadcasting field for nearly 40 years, I'd like to point out that we live in a time when the news media is seemingly obsessed with "branding." This marketing strategy involves creating a differentiated name and image — often using a tagline — in order to establish a presence in people's mind. In recent years in the field of astronomy, for example, we've seen annular eclipses — those cases when the moon is too small to completely cover the disk of the sun — become branded as "Ring of Fire Eclipses." A total eclipse of the moon — when the moon's plunge through the Earth's shadow causes the satellite to turn a coppery red color — is now referred to as a "Blood Moon." 
 
 When a full moon is also passing through that part of its orbit that brings it closest to Earth — perigee — we now brand that circumstance as a supermoon. That term was actually conjured up by an astrologer back in 1979 but quite suddenly became a very popular media brand after an exceptionally close approach of a full moon to Earth in March 2011. It surprises me that even NASA now endorses the term, although it seems to me the astronomical community in general shies away from designating any perigee full moon as "super." 
 
 Then there is Blue Moon. This moniker came about because a writer for Sky & Telescope Magazine misinterpreted an arcane definition given by a now-defunct New England Almanac for when a full moon is branded "blue," and instead incorrectly reasoned that in a month with two full moons, the second is called a Blue Moon. That was a brand that quietly went unnoticed for some 40 years, until a syndicated radio show promoted the term in the 1980s and it then went viral. So now, even though the second full moon in a month is not the original definition for a Blue Moon, in popular culture we now automatically associate the second full moon in a calendar month with a Blue Moon. 
 
 So are you ready for yet another lunar brand? The newest one is Black Moon. 
 
 Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.
– 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.)