July's second full moon is set to appear on July 31, and will be called the blue moon. Will people get to see a blue-colored moon at the end of this month?

The phrase "once in a blue moon" is commonly used to describe a very rare occurrence. However, it appears that a "blue moon" is not rare after all, since it happens every two to three years, according to Earthsky.org.

So what is a blue moon? And does the moon really appear blue?

The term blue moon is used to describe lunar events in which full moons appear twice in one calendar month.

For this year, a full moon appeared last July 2 and another one — which will be the blue moon — will appear on July 31.

The term is also given to describe the third full moon occurring in a complete season (spring, summer, fall, and winter) that has four full moons.

Typically, a complete season has only three full moons, and the third one, if a fourth full moon arises, will be called the blue moon.

But which of the two definitions is the right one? According to experts, the seasonal definition is more accurate, but both definitions are accepted.

The monthly definition came from an article by James Hugh Pruett, an astronomer hobbyist, back in the 1880's. The article appeared in the magazine Sky and Telescope in 1946, as reported in TimeandDate.

He miscalculated and stated that the second full moon in a month is the blue moon.

His claim was immediately disproved with a correction printed afterward. But it was too late because his definition was popularly used due to its simple explanation.

Instead of labeling it as a mistaken claim, the monthly definition was called a "second definition" because of its common usage.

Meanwhile, a moon that appears blue in color does happen but it's a very rare sighting. There is such an occurrence and it happens if the atmosphere is filled with small particles like dust or smoke.

A blue-colored moon happened back in 1883, during the eruption of the Indonesian volcano Krakatoa, when ash filled the atmosphere.

Other weak eruptions have also caused a blue-colored moon. For instance, it was reported that moons turned blue during the eruption of Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

National Aeronautics and Space Administration (NASA) advises people to get out during sunset on July 31, to look to the direction of the east and witness the blue moon.