Fox News Weather Center

What is a blue moon?

While the phrase 'once in a blue moon' is well-known, many may not realize the true definition of an astronomical blue moon.

"The second full moon in a given calendar month is coined a ‘blue moon,'" AccuWeather Senior Meteorologist David Samuhel explained.

Blue moons typically occur once every few years.

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Samuhel added that a second, lesser known definition of a blue moon is the third of four full moons that occurs during a single astronomical season.

According to EarthSky, blue moons under the second definition are rare, which is how the term ‘once in a blue moon' originally came about.

Despite its name, a blue moon is rarely the color blue.

"The term ‘blue moon' actually has nothing to do with the color of the moon," Samuhel said.

Blue moons are usually the same gray and white color of a regular moon.

Only in rare cases can the moon be tinted blue.

It usually takes a volcanic eruption or large wildfire for such a spectacle to take place, according to NASA.

The dust and smoke particles that are released into the air during such events are usually the perfect size for blue light to pass through the atmosphere. This creates a blue filter in front of the moon, thus making the moon appear blue to the human eye.