The enigmatic blue moon made an appearance on Friday morning, but amateur astronomers shouldn't feel too bad if they thought the phenomenon looked just like every other full moon.

That's because the blue moon isn't actually blue. In fact, it's just a regular full moon, but one that appears an unusual amount of times in the month or season.

Here are five things to know about the blue moon and its history:

Friday's moon was considered "blue" because it was the second full moon of the month.

According to NASA, the commonly-accepted definition of a blue moon is the second full moon in a month.

This year, a full moon shone on July 2, and a second one appeared at 6:43 a.m. EDT on July 31.

But Shakespeare wouldn't have called Friday's moon blue.

Historically, the phrase blue moon has had a number of different meanings.

While it is impossible to know who invented the term, Memorial University Professor of Folklore Philip Hiscock said it dates back at least to Shakespearean times.

"Like so much folk speech, it probably moved through many ears and mouths before anyone started thinking of it as a specific phrase," Hiscock said in an email to CTVNews.ca.

When it emerged in the 1500s, Hiscock said, the term blue moon referred to something "absurd."

"The very earliest uses of the term were remarkably, like saying the Moon is made of green cheese," Hiscock wrote in a 2012 article.

In the following centuries, however, the term "once in a blue moon" shifted to refer not to the ridiculous, but to the rare.

An official definition of the blue moon emerged in the 1937…but it was too confusing.

According to NASA, the Maine Farmer's Almanac tried to define a blue moon in its 1937 edition. But the complicated explanation -- which depended on dates from the Christian calendar, the timing of the season, and tropical years -- ended up confusing even astronomers.

Alternatively, Geoff Chester of the U.S. Naval Observatory told The Associated Press on Thursday that the blue moon could also refer to the fourth full moon in an astronomical season. As Friday's moon was the second of the summer, it wouldn't count as blue under that definition either.

Blue-coloured moons do exist , but they need the help of a volcanic eruption or forest fire.

When ash and smoke rise in the atmosphere, they can create a blue filter and change the way the moon's light appears on Earth. When this happens, NASA says, the moon can actually appear to be tinted blue.

This phenomenon was spotted after volcanic eruptions at Mount St. Helens in Washington state, and Indonesia's Krakatoa volcano, the space agency says. Blue moons were also seen over Canada in 1953, when there was a massive muskeg fire in Alberta, and can occur when there are summer forest fires.

Under the current commonly-accepted definition, the next blue moon will rise in January 2018.

If blue moons are the second full moon in a month, then Earth will see another one on January 31, 2018.

For those following Chester's alternate definition, however, the next blue moon will actually be in May 2016.

For Hiscock, the changing definition reflects the "divorced-from-nature" way modern people look at the sky.

While moon cycles once determined how people measured time, that's no longer the case in today's Western societies.

"In the modern world we hang things, not on the moon's cycle, but on dates on a calendar," Hiscock said.