Friday’s ‘blue moon’ will look like any other full moon

Published 7:51 pm Tuesday, July 28, 2015

A blue moon will occur Friday, but it has nothing to do with beer, and there probably won’t be anything blue about it.

But it will be a second full moon in the calendar month of July 2015, which is commonly referred to as a “blue moon.”

The rare celestial event is more of a calendar flub caused by mismatched calendars than anything else.

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The western calendar doesn’t fully line up with the lunar calendar, because the time between two full moons is actually about 29.5 days. According to, blue moons occur about every 33 months, approximately every three years.

The last blue moon was in August of 2012, and the next won’t occur until 2018.

It is possible for two blue moons to occur in a single calendar year.

It last happened in 1999, according to, and will again occur in 2018 and 2037. The double blue moons occur when there are two full moons during the month of January and two full moons during March and no full moons in February.

This definition of “blue moon” is relatively modern, stemming from an article from the March 1946 issue of “Sky and Telescope” magazine in which the author misinterpreted the previous definition that also had nothing to do with the moon actually being blue.

A “blue moon” was once defined as the third full moon in a series of four within a season.

One season – winter, spring, fall, summer – is typically three months long with three full moons. The third in the season would be referred to as a “blue moon.” According to, a blue moon by this definition happened in August 2013 and will again occur in May 2016.

The most rare of all the blue moons is a moon that actually appears blue in color. Unusual sky conditions and dust particles just the right size can create them, but they aren’t predictable like the other types of blue moons.

Volcanic eruptions and major forest fires that produce a lot of smoke and dust can cause blue coloring.

According to, this is the more widely accepted source of the phrase, “once in a blue moon,” as the idiom means “rarely” or “absurd.”