Blue Moon Definitions

What is a blue moon?

  • Definition 1: The currently accepted meaning is that a blue moon is the second full moon in a single month which has two full moons. Usually a month has one full moon, 12 in total for a year. Every two or three years there are 13 full moons in a year.
  • Definition 2: Said to be the original meaning, a Farmers’ Almanac blue moon is the third full moon of a season (three months) that has four moons instead of three.
  • It takes 354.37 days for the moon to go around the earth 12 times, a lunar year. This does not match the same time it takes for the earth to go around the sun of 365.24 days, an earth year. This means very few years there is an extra moon that makes up this difference.
  • It is a common myth that a blue moon is called blue because of its colour – though the moon can turn a blue colour with certain volcanic ash in the air – which happens very rarely – perhaps, once in a blue moon, you would say?? The moon is more likely to turn red than blue with dust/ash particles in the air.
  • Blue moons can have a different occurrence on each side of the international date line. For example, the USA had one blue moon in 2009 (Dec 31) and Australia did not have any. But in 2010 Australia has two blue moons (Jan 31 and Mar 31) while the USA has none. This is because the USA is a day behind Australia and so a full moon in the last day of a month in the USA is a full moon on the first day of the next month in Australia and New Zealand. (See dates here)
  • It is possible to have a year which has two months with a blue moon, and with that, another month that has no full moon. There are only ever a maximum of 13 moons in a year and a minimum of 12.
  • A year with no blue moons is called a 'metonic' year.