What Is a Blue Moon, Really?

"Once in a blue moon" means something rare. But a blue moon is a real calendar event, and despite the name, the moon almost never looks blue. Here is what it actually means.

The common definition

A blue moon is usually the second full moon in a single calendar month. Full moons come about every 29.5 days, and most months are a little longer than that, so now and then a month gets two full moons. The second one is the blue moon. It happens roughly every two to three years.

The older definition

There is an older, seasonal version too: the third full moon in a season that has four full moons instead of the usual three. This is where the term started. The "second full moon in a month" version came later, from a magazine mix-up that stuck. Both are accepted now.

Does the moon ever look blue?

Yes, but not because of the calendar. The moon can take on a genuine blue tint when the air is full of fine particles of just the right size, usually from a big wildfire or a volcanic eruption. Those scatter red light and let blue through. It is rare and has nothing to do with blue moons by name.

Is there anything special to see?

Honestly, a blue moon looks exactly like any other full moon. The fun is in the timing, not the view. It is a good excuse to go out and look up.

You can see every full moon coming up on moontrack, or read about the names of the full moons.