You’ve heard, and maybe even used, the expression “once in a blue moon.” But what exactly does it mean? What is a blue moon?
Simply, a blue moon refers to a second full moon in a single calendar month.
Since the moon takes 29.5 days to complete a lunar cycle each calendar month it’s rare that two full moons can be squeezed into a single month. But every so often—or, to be exact, every 2.7 years—they can!
Normally, there is only one new moon and one full moon per months—in other words, a total of 12 full moons per solar calendar year. In blue moon years, there are 13 full moons on the roster, a “bonus moon” if you will.
This can only happen in a month that has 30, or usually 31, days. So we will never have a blue moon in February.
In the Farmer’s Almanac, a blue moon means there are four full moons in a season, rather than three. Since this almanac was historically used for planting, hunting and fishing, it divides the year up by seasons.
So…the moon doesn’t turn blue?
Nope! It’s a colloquial expression of uncertain origin that basically means “never.” (Once in a blue moon…when pigs fly…you get it.)
That said, the moon CAN actually appear blue at times due to atmospheric effects. For example, if there’s been a forest fire or volcano, it can impact the way light bends and refracts. The smoke and ash can cause red light wavelengths to scatter, allow other colors to pass through the Earth’s atmosphere and be reflected on the surface of the moon. This can cause the moon to appear tinted with a blue, green or purple-ish hue.
How is a blue moon different from a blood moon?
The blue moon is not to be confused with a “blood moon.” A blood moon happens at a lunar eclipse, when the Earth passes directly between the Sun and the moon, throwing shadows on the moon and plunging it into darkness. As the sunlight moves through the Earth’s atmosphere it bends into short-wavelength shades of reddish-brown and orange that are reflected on the moon’s surface, resembling dried blood. Not exactly as romantic as it sounds, but that’s the science of it!
On January 31, 2018, the world experienced a full moon that was both a blue moon AND a blood moon. That night, the Leo lunar eclipse arrived as the second full moon of the month AND a supermoon—the first time all three of these events occurred simultaneously in 150 years!