It’s the first time since 2012 and it won’t happen again until 2018.
So you should probably get outside and look up tonight. Just don’t be surprised when the moon you see isn’t particularly different from the one you’re used to seeing (though it might appear to be slightly bigger to the keen moon-watching eye).
According to The Weather Network, there are actually three definitions for modern term ‘blue moon’, but in this post we’re forcing on the third and most common definition:
1. According to the Maine Farmer’s Almanac a “blue moon” is the third full moon that occurs during an astronomical season with four full moons (normally there are three full moons during a season). There are three full moons during Summer 2015 – July 2, July 31 and August 28 – so this full moon doesn’t satisfy that condition.
2. According to Pagan beliefs, the “blue moon” is the second full moon that occurs during an astrological month – the period when the Sun is passing through a particular sign of the zodiac. As the July 2 and July 31 full moons occur when the Sun is in different signs (Cancer and Leo), it also doesn’t qualify for this type of “blue moon.”
3. The most common modern definition is what we’re seeing on Friday – the second of two full moons that occur within the same calendar month. This is based on a misinterpretation of the Farmer’s Almanac definition, and was printed in the March 1946 issue of Sky & Telescope. Regardless of being wrong, it has still become the most popular definition.
And know you know.