A “blue moon is not blue.” That’s the first thing you should know when you hear people talking about it. Blue moon is a nickname usually given to the second full moon that happens in a single calendar month. It’s not a scientific definition and it has nothing to do with the color of the full moon. Since the moon cycle is about 29 and 1/2 days, every few years there will be two full moons that happen in one month. Here’s a calendar of two full moons happening in one month.
While it is possible for any full or bright moon to take on a blue tint, based on a dust particles in the Earth’s atmosphere that refract light, the moon is commonly tinted orange or red when it is low on the horizon, for the same reason that the sun is more orange or red when it rises and sets- Light is passing through greater amounts of air, and if the air has a lot of particles like dust, ash, soot, smoke, or even just water vapor, the shorter wavelength colors are filtered out, and the yellows, oranges, and reds can make it through.
There are many nicknames for full moons which are based on folklore, and on different cultures around the world. Those nicknames don’t describe the moons color either!
Read the full story of how the confusion started over what a “blue moon” is from the Library of Congress.
Some people also use “blue moon” to mean the third full moon out of four full moons that happen in a season. Earthsky.org explains that one.
As an expression, once in a blue moon, simply means something that doesn’t happen too often.
Don’t fall for the hype on social media! A “Pink Moon” is not pink and a “Green Moon” is a hoax!
Each full moon has nicknames from folklore often based on lifestyles of people in the past. Moon nicknames are given by different cultures, and sometimes religions, but don’t expect the moon to take the color of the nickname. A “pink moon” is not pink. You probably can guess that a “hunter’s moon” does not look like a hunter either!
Here’s a list of just some of the common moon nicknames from Earthsky.
It is possible for the moon to be tinted by dust, ash, soot and other particles in the atmosphere, especially when it is low in the sky. Just like the sun, the moon is more likely to take on a red or orange tint when it is low, and the sun is near the horizon at the same time.
So what about a “Green Moon”? That is simply a social media hoax. The moon will not look green. Here’s the history of that urban legend from Snopes.