Saturday night’s full moon will be the last blue moon until 2020. It will not, however, actually look blue. Unless something has gone terribly wrong.
The moon only appears blue when light is refracted by an abnormal amount of particles in the air. It takes something like smoke from a major forest fire or the ash cloud from a volcanic explosion for the moon to look really blue.
Not only isn’t a blue moon blue, it’s not as rare as the phrase “once in a blue moon” would lead you to believe. A blue moon is the second full moon in a single month, and on average one happens every 2.66 years, but 2018 has been an unusual year for blue moons.
In the early hour of Jan. 31, a super blue blood moon was visible in North American skies. It was a supermoon because the moon was at the closest point it gets to Earth in its orbit (so it looked really big). It was a blood moon, because there was also a lunar eclipse and the Earth’s shadow made the moon appear red-ish. It was a blue moon for the standard reason. And it really was rare.
The last super blue blood moon visible from North America prior to 2018 happened in 1866.
Using “once in a blue moon” to describe something as rare seems to date back to the 16th century. Its earliest known appearance in print occurred in England in 1528, in a satiric aside in Bishop William Barlow’s Treatyse of the Buryall of the Masse:
Yf they saye the mone is belewe,
We must beleve that it is true.
Even though Saturday’s blue moon won’t be super or bloody, it will still be a fairly rare occurrence, because two blue moons in a single year won’t happen again until 2037.