Blue moon, 2nd full moon in July, arrives early Friday

For the second time in July, skywatchers will be able to look up to a full moon. Here are six things you need to know about the lunar event with a somewhat misleading name.

2nd full moon in a calendar month occurs once every 2½ years on average

A United Airlines passenger plane crosses the waning gibbous moon, one day after a full moon on July 2, in Whittier, Calif. There are two full moons this month. (Nick Ut/Associated Press)

An astronomical phenomenon that occurs once in a blue moon is happening early Friday morning — a blue moon.

For the second time in July, skywatchers will be able to look up to a full moon. Here are six things you need to know about the lunar event with a somewhat misleading name.

What is a blue moon?

Whenever there are two full moons in a calendar month, the second one is called a "blue moon," according to popular definition. The first full moon this month appeared on July 2, and a second one — the "blue" one — is happening Friday. While the moon will rise just before sunset Thursday, it won't be full until 6:43 a.m.

Is the blue moon actually blue?

Short answer: yes and no.

Most blue moons will look pale gray and white, just like on any other night.

"Squeezing a second full moon into a calendar month doesn't change its colour," NASA says.

However, the moon can turn blue on rare occasions, such as a volcanic eruption, NASA says.

In 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded. The culprit was the plumes from the eruption, which were filled with particles one micron wide. Particles of this size act like light filters and scatter red light, allowing blue light to pass through.

In a similar fashion, forest fires — which produce smoke with micron-sized particles — can also affect the way we see the moon. NASA recalled a famous incident in September 1953 when a muskeg fire burned in Alberta. Clouds of smoke containing micron-sized oil droplets made the sun look lavender and the moon blue from North America to England.

The moon could also look red, thanks to the abundance of aerosols in the atmosphere which scatter blue light, allowing red light to go through, NASA says.

 "For this reason, red blue moons are far more common than blue blue moons," according to NASA.

Why is it called a blue moon if it's not blue?

The popular definition of blue moon came about after a writer for Sky & Telescope magazine in 1946 misinterpreted the Maine Farmer's Almanac and labelled a blue moon as the second full moon in a month. In fact, the almanac defined a blue moon as the third full moon in a season with four full moons, not the usual three. Though Sky & Telescope corrected the error decades later, the definition caught on.

How often does a blue moon happen?

Most years have 12 full moons. A blue moon, however, occurs every 2½ years on average.

The last blue moon appeared in August 2012. The next blue moon will not happen until January 2018.

What's the blue moon's importance?

Blue moons have no astronomical significance, says Greg Laughlin, an astronomer at the University of California, Santa Cruz.

"Blue moon is just a name in the same sense as a 'hunter's moon' or a 'harvest moon,'" Laughlin previously told The Associated Press.

"It's not blue. It's not even rare," astrophysicist Neil deGrasse Tyson said on Twitter on Wednesday.

When can I see it?

On Thursday evening, the moon will rise just before sunset and be full at 6:43 a.m. Friday ET, according to Sky & Telescope magazine.

What colour is the blue moon in your area? Share your photos with us on Twitter @cbctrending or email us.

With files from The Associated Press