why do we have a blue moon

In recent years, people have been using the name Blue Moon for the second of two full moons in a single calendar month. An older definition says a Blue Moon is the third of four full moons in a single
season. Someday, you might see an actual blue-colored moon. The term once in a blue moon used to mean something rare. Now that the rules for naming Blue Moons include several different possibilities, Blue Moons are pretty common! We just had a Blue Moon on January 31, 2018. It was a supermoon, too, and underwent a total eclipse. But another Blue Moon is coming right up. TheyБre both Blue Moons by the monthly definition of the term: the second of two full moons to fall within a single calendar month. The first Blue Moon was on January 31, and the second on. We havenБt had a year with two Blue Moons since 1999 and wonБt have one again until January and March, 2037. Meanwhile, the month of February 2018 has no full moon at all. The Maine FarmerБs Almanac defined a Blue Moon as an extra full moon that occurred in a season. One season Б winter, spring, summer, fall Б typically has three full moons. If a season has four full moons, then the third full moon may be called a Blue Moon. There was a Blue Moon by this definition onб , another on August 20-21, 2013, and another on May 21, 2016. The next seasonal Blue Moon (third of four full moons in one season) will take place May 18, 2019. Very rarely, a monthly Blue Moon (second of two full moons in one calendar month) and a seasonal Blue Moon (third of four full moons in one season) can occur in the same calendar year. But for this to happen, you need 13 full moons in one calendar year and 13 full moons in between successive December solstices. This will next happen in the year 2048, when a monthly Blue Moon falls on January 31, and a seasonal Blue Moon on August 23.


Using the name Blue Moon to describe the second full moon of a calendar month is now the best-known and most popular definition. By this definition, there was a Blue Moon on July 31, 2015, and then Б of course Б the recent one on January 31, 2018. The time between one full moon and the next is close to the length of a calendar month. So the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every two to three years, so these sorts of Blue Moons come about that often. The idea of a Blue Moon as the second full moon in a month stemmed from the March 1946 issue of Sky and Telescope magazine, which contained an article called БOnce in a Blue MoonБ by James Hugh Pruett. Pruett was referring to the 1937 Maine FarmerБs Almanac, but he inadvertently simplified the definition. He wrote: Seven times in 19 years there were Б and still are Б 13 full moons in a year. 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. Had James Hugh Pruett looked at the actual date of the 1937 Blue Moon, he would have found that it had occurred August 21, 1937. Also, there were only 12 full moons in 1937. You need 13 full moons in one calendar year to have two full moons in one calendar month. However, that fortuitous oversight gave birth to a new and perfectly understandable definition for Blue Moon. EarthSkyБs happened upon a copy of this old 1946 issue of Sky and Telescope in the stacks of the Peridier Library at the University of Texas Astronomy Department in the late 1970s. Afterward, she began using the term Blue Moon to describe the second full moon in a calendar month on the radio. Later, this definition of Blue Moon was also popularized by a book for children by Margot McLoon-Basta and Alice Sigel, called KidsБ World Almanac of Records and Facts, published in New York by World Almanac Publications in 1985.


The second-full-moon-in-a-month definition was also used in the board game Trivial Pursuit. Today, it has become part of folklore. As the folklorist Philip Hiscock wrote in his comprehensive article Old folklore it is not, but real folklore it is. Can a moon be blue in color? Yes, but itБs very rare to see a blue-colored moon. You need unusual sky conditions Б certain-sized particles of dust or smoke Б to create them. Blue-colored moons arenБt predictable. So donБt be misled by the photo above. The sorts of moons people commonly call Blue Moons arenБt usually blue. For more about truly blue-colored moons, click. Bottom line: A blue-colored moon is rare. But folklore has defined two different kinds of Blue Moons, and moons that are Blue by name have become pretty common. The next Blue Moon is March 31, 2018. Blue Moon: Is There Really Such a Thing? The saying "once in a blue moon" has nothing to do with color. Instead it refers to the relatively rare occurrence of what is called a "blue moon". (However, an actual blue-colored moon could occur from atmospheric effects such as a big forest fire on Earth causing a lot of haze. This has happened in the past! ) The definition of a Blue Moon has changed over the years (see below). However, the commonly accepted definition of a Blue Moon is simple: it is a full moon that rises twice in one month. It happens because sometimes the number of days in a calendar month are greater than the cycle of the moon, which is 29. 5 days. This can cause the moon to rise twice in one month, near the first and the last days of the same month. It is said that blue moons happen every 33 months or about every 3 years.


This relatively rare occurrence has spawned the saying "once in a blue moon". A Blue Moon cannot happen in February because the calendar month never has enough days. Furthermore, sometimes the occurrence of a blue moon depends on your time zone. During 1993 a blue moon occurred in either in August or September depending on where you lived. If you lived east of the line that runs through the Atlantic Ocean, the blue moon occurred in September, but west of that line it occurred in August. Origins of The Definition of a Blue Moon A year can be divided into quarters, or seasons. In most years, each season contains 3 full moons. Just as the lunar cycle causes some months to have 2 full moons, some seasons will have an extra full moon. The Farmer's Almanac calls the third full moon of any season containing four in total a "Blue Moon". However, a series of cascading misinterpretations and unfounded assumptions led to a change in definition. A 1943 article followed by a 1946 article in the magazine Sky and Telescope essentially declared the second full moon in the same month to be a "Blue Moon" in reference to data found in an edition of the 1937 Maine Farmers Almanac. But the interpretation was incorrect. Over the course of decades, this interpretation was repeated until it became "fact". You can read the entire story in detail on the Sky and Telescope website. Note that a season isn't exactly a quarter of a year. It is slightly altered because of the archaic Christian ecclesiastical calendar. This was the calendar used by the Christian church to determine the exact date for certain holidays such as Easter, and it did not use the same lunar cycle that modern science recognizes. Upcoming Blue Moon Dates These dates are based on the more well-known definition of a Blue Moon (the second full moon in a calendar month).

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