What is a blue moon? And how often does one occur?

Answer 1

This is typically the occurance of a 13th full moon in a year. There are generally 12 full moons in a year. But since the lunar cycle is approximately 30 days to a lunar month, occasionally there will be a 13th full moon in a year.

There is a theory that a blue moon is the second full moon of a month. That would be a blue moon, only if the following month also had a full moon. Otherwise, it is just a cyclical adjustment for the "irrgular" solar months (the days in a solar month are not all the same.) So, there could be a full moon in the early days of a month and another at the end of that month. If the following month is short (like february) then there could be no full moon on that month. So, this is not really a blue moon.

Answer 2

The astronomical definition of a Blue Moon is the second Full Moon to occur in one month. On average a Blue Moon occurs about every two and a half years.

The time interval between Full (or New) Moons is about 29.5 days, this period being called a Lunation. (A Blue Moon in February is thus not possible, months with 31 days will be more likely to 'contain' a Blue Moon.) Given the relative infrequency of such an event has lent us the phrase "Once in a Blue Moon" - not that often.

But, that is the astronomical definition. Each Full Moon of a month is named after something agricultural (Hunters Moon, Harvest Moon etc.) and according to folklore Moons are assigned other names as well.

Finally, another definition is more literal insomuch that the Moon really can appear blue due to atmospheric pollution, typically from volcanic eruptions.

Answer 3

A blue moon is the second full moon in a calendar month.

If we take the time between two full moons to be the average synodic month of M = 29.530588853 days we see that a blue moon cannot occur in a month of 28 or 29 days.

The probability that a month of 30 days has a blue moon in it is the probability that the beginning of the month occurs less than 30-M days before a new moon. That probability is 30/M-1, which is about 1.59 %.

Similarly, the probability that there's a blue moon in a 31-day month is 31/M-1 or about 4.976 %.

To find out how often a blue moon occurs, you could compute a complicated weighted average of the above probabilities over a complete 400-year Gregorian calendar cycle, but there's a better way, which is more straightforward:

A 400-year Gregorian cycle represents 146097 days (1 "long" century of 36525 days and 3 "short" ones of 36524 days). This is about 4947.31 lunar months and exactly 4800 calendar months...

Therefore, on average, there will be 4947.31-4800 = 147.31 blue moons in 400 years (4800 calendar months = 146097 days).

The average time between two blue moons is therefore 146097/147.31 = 991.76 days (a little less than 3 years).

Of course, this is only a LONG-TERM average; the actual time between two actual consecutive blue moons varies widely. On the average, "once in a blue moon" is thus (more or less) "once in a thousand days".

Answer 4

That's when u get two full moons in the same month. The time between two full moons is about. 29.53 days so u can see that if a full moon occured on the first day of any month except February, u could get a second (blue) moon before the month ended.

February can't have blue moons even in a leap year because 29.53 days can't fit into 29 days

Answer 5

The trendy definition of "blue Moon" as the second full Moon in a month is a mistake.

Recent decades have seen widespread popular embrace of the idea that when a calendar month contains two full Moons, the second one is called a "Blue Moon." The unusual pattern of lunar phases in early 1999 — two full Moons each in January and March, and none at all in February — triggered a groundswell of public interest. Countless newspapers and radio and TV stations ran stories about Blue Moons.

In an article "Once in a Blue Moon", folklorist Philip Hiscock traced the calendrical meaning of the term "Blue Moon" to the Maine Farmers' Almanac for 1937. But a page from that almanac belies the second-full-Moon-in-a-month interpretation.

With help from Margaret Vaverek (Southwest Texas State University) and several other librarians, we have now obtained more than 40 editions of the Maine Farmers' Almanac from the period 1819 to 1962. These refer to more than a dozen Blue Moons, and not one of them is the second full Moon in a month. What's going on here?

Answer 6

A "blue moon" is the situation when you get two full moons in a single month -- the second full moon is called a "blue moon". So, on average, there's a blue moon once every 2-3 years.

On average, each century will have ~41 blue moon months. The last was May 2007 (on the east coast of the U.S. See here for more: http://www.obliquity.com/astro/blue2007.html)

Answer 7

A blue moon is a full moon twice in one month

On average there will be 41 months that have blue moons in a centuary, so you could say that once in a Blue Moon actually means once every two-and-a-half years.

We just had one last may.

Answer 8

It's a Song

Answer 9

once in a blue moon i supose.