Picture of a Full Moon taken from the ISS
Question: If a Blue Moon is a rare event, why were there two Blue Moons in 2010?
Answer: A Blue Moon is the name given to the second Full Moon that falls in a calendar month. It is a relatively rare event, happening on average every two-and-a-half years. It is possible to have two Blue Moons in the one year but this is an even rarer event!
Most months have one Full Moon – after all, the word month derives from the word moon. This is because months are approximately as long as the time it takes to go from one Full Moon to the next. On average this is 29.5 days, a period known as the synodic month.
However calendar months aren't exactly as long as the synodic month. Most are a bit longer: 30 or 31 days. This means that if a Full Moon falls within the first day or two of a month, then a second Full Moon may fall right at the very end of the month, giving that month not one but two Full Moons, thus creating a Blue Moon.
So how do you get two Blue Moons in the same year? February, with 28 or 29 days, is slightly shorter than a synodic month and so can never have two Full Moons – and may even have no Full Moon. So if a second Full Moon falls at the end of January (making January a Blue Moon month) then the next Full Moon can skip February and fall at the start of March and again at the end of March (making March a Blue Moon month also). This is what happened in 2010 – the Full Moon occurred on January 1 and 30, and March 1 and 30 – and is the only way that a year can have more than one Blue Moon. On average it happens once every 19 years – which is even more infrequently than "once in a Blue Moon"!
In fact the phrase "once in a Blue Moon", meaning an unusual event, was in use before it had applications to astronomy. It dates back to a 16th century poem criticising church authority: "If they say the Moon is blue/We must believe that it is true". (It is possible that this couplet included a pun on the way that the Church calculates the date of Easter using an approximate method, rather than according to the true date of the Full Moon.) In this original meaning, a Blue Moon meant something ridiculous and unbelievable. Later the meaning shifted to meaning something rare but not impossible.
The use of the term Blue Moon as a folk name for the astronomical event started in the USA. Native American tribes had special names for each of the Full Moons in a year, and American settlers adopted this custom, changing some of the names. In the 19th century the Farmers' Almanac started publishing lists of dates of the various Full Moons. However when there was an extra Full Moon in the year they had to decide which was the 'extra' Moon so as to determine which name to use for each.
Originally the Farmers' Almanac decided that the third Full Moon in the season with four Moons was the extra, or Blue Moon. (The last Full Moon in the season of three months was always given its usual name.) However in 1946 an article in Sky & Telescope misunderstood this, and instead gave the modern meaning: that a Blue Moon was the second Full Moon in any month with two. This misunderstanding spread very rapidly, and today is the only meaning used.
The new meaning could take hold so quickly largely because Blue Moon is not an official astronomical term at all; it is just a popular term. Nor does it refer to the apparent colour of the Moon. Smoke and dust from bushfires or volcanoes can change the observed colour of the Moon, making it appear reddened when low on the horizon or bluer when high in the sky, but these events occur on Earth, and are not caused by astronomical cycles!