2012 8月底（8月31日）你將有機會親眼看到所謂的「藍月亮」，但什麼是「藍月亮」？也許你聽過這個：「once in a Blue Moon(千載難逢的機會)」，到底出現藍月亮的幾率有多小，多特別呢？
如果講到藍月亮的話，有幾個必須要清楚的信息。一，藍月亮並不是藍色的，其實跟平常滿月時的月亮顏色差不多。二，對於藍月亮的定義一直是有爭議的。比如，如果你是一個純粹主義者，下個月出現的藍月亮並不是真正意義上的藍月亮。
藍月亮屬罕見天文現象，但也並非極不尋常。一般來說，會按照時間順序和月相變化將四季和公曆月儘可能的互相匹配好。一年分為四個季節，那麼每個季節有3次滿月；或者一年分為12個月，一個月1次滿月。但是大自然的時間總是會和人類設定的時間有一定的不同。因此，有時候一個季節裡有4次滿月而不是3次，那麼一年就會有13次滿月。這種現象大約每隔2.7年出現一次。
按照這樣的來的話，一個季節中的第4次滿月被稱為藍月亮。但是，如果根據這個定義，2012是沒有藍月亮的。夏季——即6月21日至9月22日，這個季節只有3個滿月啊，為什麼大家都說下個月會有藍月亮呢？原來因為在1946年時，藍月亮的定義被拓寬了，也就是說，只要在一個月內出現兩次滿月也可以。這樣的話，這一個月的第二次滿月亮就被稱為藍月亮。
這個新定義使藍月亮一年就可以出現一次。但是更有趣的是，也有可能一年出現2次藍月亮。在1999年時，一月份有2個滿月，三月份有兩個滿月，而二月份一個滿月都沒有。這樣的怪異天象也引起了地理位置和時區的變化。今年，由英國倫敦的0度經線開始，加9個小時(東九區，日本)將在9月經歷2個滿月(9月1日一次，9月30日一次)，這意味著他們的藍色月亮將會出現在9月底而不是8月底。
2012年的8月2日11時25分為滿月時間，8月31日21時56分為滿月時間，那麼8月份就有2個滿月。因此我們可以在2012年8月2日11:25（看到第一次滿月），8月31日21:56（看到藍月亮）
The next Blue Moon is August 31, 2012
According to modern folklore, a Blue Moon is the second full moon of a calendar month. Learn how this name came to be, in this post.
August 2012 is a month with two full moons. And, by popular acclaim, that means it’s a Blue Moon month – but it’s Blue in name only. That’s because a Blue Moon is sometimes defined as the second full moon in a calendar month. The first full moon is August 1. The second full moon is August 31, 2012.
There are two more definitions for Blue Moon. It can be the third of four full moons in a single season. Or, someday, you might see an actual blue-colored moon.
It’s very rare that you would see a moon that’s actually blue in color. This photo was created using special filters. The August 31, 2012 Blue Moon will be called Blue because it is the second full moon of a month.
It’s very rare that you would see a blue-colored moon, although unusual sky conditions – certain-sized particles of dust or smoke – can create them. Blue-colored moons aren’t predictable. So don’t be misled by the photo above. The sorts of moons people commonly call Blue Moons aren’t usually blue. For more about truly blue-colored moons,
Now on to folklore’s Blue Moons. Every month has a full moon, and, most of the time, the names for full moons coincide with particular months or seasons of the year. By either definition, the name Blue Moon accounts for times when there happen to be more full moons than is convenient.
Blue moon as second full moon in a month. In recent decades, many people have begun using the name Blue Moon to describe the second full moon of a calendar month.
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 2-3 years, so these sorts of Blue Moons come about that often.
When is the next Blue Moon, according to this first definition? August 31, 2012.
Another beautiful image by our friend Jv Noriega – the moon among fast-moving clouds.
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 “Once in a Blue Moon” by James Hugh Pruett. Pruett was using a 1937 Maine Farmer’s Almanac, but he simplified the definition. He wrote:
Seven times in 19 years there were — and still are — 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.
EarthSky’s Deborah Byrd 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 “Kids’ 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.
Can there be two blue moons in a single calendar year? Yes. It last happened in 1999. There were two full moons in January and two full moons in March and no full moon in February. So both January and March had Blue Moons.
The next year of double blue moons is coming up in 2018.
What most call a Blue Moon isn't blue in color. It's only Blue in name. This great moon photo from EarthSky Facebook friend Rebecca Lacey in Cambridge, Idaho.
Blue moon as third full moon of four in a season. The Old Farmer’s Almanac defined a Blue Moon as an extra full moon that occurred in a season. One season – winter, spring, fall, summer – typically has three full moons. If a season has four full moons, then the third full moon may be called a Blue Moon.
The next blue moon by this definition will fall on August 21, 2013.
In recent years, a controversy has raged – mainly among purists – about which Blue Moon definition is better. The idea of a Blue Moon as the third of four in a season may be older than the idea of a Blue Moon as the second full moon in a month. Is it better? Is one definition right and the other wrong? After all, this is folklore. So the folk get to decide, and, in the 21st century, both sorts of full moons have been called Blue.
As the folklorist Phillip Hiscock wrote in his comprehensive article Folklore of the Blue Moon: Old folklore it is not, but real folklore it is.
So enjoy Blue Moons!
Bottom line: A blue-colored moon is rare. But folklore has defined two different kinds of Blue Moons. A Blue Moon can be the second full moon in a month. Or it can be the third of four full moons in a season. The full moon of August 31, 2012 will be considered a Blue Moon.
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