A Blue Moon. Credit:

What is a Blue Moon?

Article Updated: 24 Dec , 2015



A lunar month takes 29.53 days. This is the amount of time it takes for the Moon to complete a cycle through all the phases, from new moon to full moon and then back to new moon again. This is very close to the length of a month in the Western calendar, which usually have 30 or 31 days. Every month usually has one of each of the phases. So a typical month will have a new moon, first quarter, full moon and last quarter moon. But every now and then, a month will have two of the same phases. When a month has two full moons, the second one is called a “blue moon”.

Blue moons are rare, and that’s where the phrase comes from, “once in a blue moon”. How rare? They always happen once every 2-3 years (2.72 years, to be exact). It’s this second moon in the month that’s considered the blue moon.

Does the blue moon actually turn blue? No. A blue moon is exactly the same color as a regular full moon – yellow.

The Moon can turn blue when there’s a certain amount of dust or pollution in the air. The extra dust scatters blue light, making the Moon appear more blue. For example, the Moon appeared blue across the entire Earth for about 2 years after the eruption of Krakatoa in 1883.

Here is a list of upcoming blue moons:

  • 2018: January 2, January 31st
  • 2018: March 2, March 31st
  • 2020: October 1, October 31st


We have an article here on Universe Today about all the phases of the Moon.

Here’s an article from NASA about blue moons.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?


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Daniel Rey M.
Daniel Rey M.
November 6, 2008 10:50 AM
An article titled “What’s a Blue Moon?” in the May 1999 issue of Sky & Telescope gives the correct explanation: “When a season contains four full Moons, the third is called a Blue Moon.” The subtitle points out in advance that this magazine is entirely to blame for the wrong definition of the expression: “A 53-year-old mistake in Sky & Telescope changed pop culture and the English language in unexpected ways.” The complete story is so tortuous that one would have to place here the entire article in order to allow a full understanding, but maybe a few passages will help. “Recent decades have seen widespread popular embrace of the idea that when a calendar month contains two… Read more »