What Is a Blue Moon, Anyway?

"Blue moon" is an idea with a rich history.
The fanciful notion of a "blue moon" has a rich history. (Image credit: Dreamstime)

Skywatchers tonight (March 31) will be treated to the second and final Blue Moon of 2018, just on the eve of Easter. What is this type of moon, and is it actually blue?

For those who wanted to see an aqua-hued orb, sorry but the term "Blue Moon" has nothing to do with the color. Instead, it refers to the appearance of an additional full moon in a given time period. Under the most popular definition today, a Blue Moon is the second full moon in a calendar month.

The moon reached its peak full phase at 8:37 a.m. EDT (1237 GMT), but the moon will still look pretty full for the following 24 hours.

Why does a Blue Moon happen? The moon changes phases as it orbits the Earth, according to the amount of light reaching it from the sun. The moon circles Earth every 27 days, but the geometry of the positioning of the Earth, sun and moon is the same every 29.5 days.

In other words, there are 29.5 days between one full moon and the next full moon. So it's unusual to have two full moons in a 30- or 31-day month. Also, February, which has only 28 or 29 days (during leap years), will never have a Blue Moon. Enjoy tonight's spectacle, because the next Blue Moon won't happen until Oct. 31, 2020.

"Most Blue Moons look pale gray and white, indistinguishable from any other moon you've ever seen," NASA stated. "Squeezing a second full moon into a calendar month doesn't change the physical properties of the moon itself, so the color remains the same."

Sometimes, however, the moon does look blue. But it has to do with atmospheric conditions — not calendar time. A famous example occurred in 1883, after the volcano Krakatoa erupted. The moon appeared blue because of all the dust the volcano lofted into the air. NASA says this particular event probably generated the phrase "Blue Moon," but volcanoes aren't the only reason a moon can look blue. Dust storms or forest fires can also generate this effect.

Incidentally, there is an older definition of a Blue Moon that refers to an extra full moon happening in a season. The Blue Moon was the third full moon during a season that had four full moons. This happens when a particular year has 13 full moons instead of the usual 12.

Even under that definition, the next seasonal blue moon won't happen until May 18, 2019. So if you have a clear night tonight, enjoy watching the moon cross the sky. You can use just your bare eyes to see the spectacle, or if you like, take out binoculars or a telescope to take a closer look at some of the craters and other features on the moon.

If the skies are cloudy, don't worry — you can always watch the Blue Moon online. Slooh will run a live webcast at 4 p.m. EDT (2000 GMT) that you can view here.

Originally published on Live Science.

Elizabeth Howell
Live Science Contributor
Elizabeth Howell is a regular contributor to Live Science and Space.com, along with several other science publications. She is one of a handful of Canadian reporters who specializes in space reporting. Elizabeth has a Bachelor of Journalism, Science Concentration at Carleton University (Canada) and an M.Sc. Space Studies (distance) at the University of North Dakota. Elizabeth became a full-time freelancer after earning her M.Sc. in 2012. She reported on three space shuttle launches in person and once spent two weeks in an isolated Utah facility pretending to be a Martian.