Is a Blue Moon Really Blue?

Is a Blue Moon Really Blue?

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What's up with the "blue moon"? The moon is not blue! It's just a nickname for the full moon in August. It is also called a "grain moon". The name may have origins from an event over a hundred years ago when the moon actually had a blue tint. Perhaps a blue tint happens occasionally which could explain why people use the expression, "once in a blue moon," to mean something that is rare.

Can a blue moon actually be blue? In theory, yes. The color of the moon depends on what type of particles or aerosols are floating in the atmosphere in the path between you and the moon. When the moon is low in the sky, you are much more likely to see a red or orange or yellow tint to it, just as you are for the sun. Longer wavelength colors have more energy to travel through thick atmosphere than shorter wavelength colors like blue.

Each full moon has a name based on history or seasons or legend. Different cultures have different names for full moons. Many people call the second full moon in a calendar month a blue moon. It's a phrase passed down through generations, in part because, it's been printed in many almanacs for decades.

There is no formal scientific definition of a blue moon. The name is based more on folklore and legend than science. Blue moons are also defined by the number of full moons in a growing season. Some almanacs list it is the third full moon in a three month season, that has a total of four full moons.

All full moons look large on the horizon. That's just how your eyes and mind process the image of something big and bright so low in the sky. To prove to yourself that the moon is no bigger when it rises than it is when it is high up, take a picture of the full moon when it is low in the sky and then take a picture a few hours later when it is higher in the sky. When you compare the pictures you'll see the moon is the same size.

Regardless of what you call it, a blue moon has no impact on weather. It's just a name. However, all moon phases do have effect on tides, based on the gravitational pull of the moon.

Read more on the history of the term "blue moon"...

EarthSky.org has a list of full moon names.

Sky and Telescope has an in-depth article on the history of "blue moon".

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