The blue moon, which refers not to color, but to rarity, will reach its full phase at 9:58 a.m. EDT Friday. The second full moon of August (the previous one occurred Aug. 1), the spectacle will reportedly not be in full view again until July 2015.
According to LiveScience, blue moons occur because our calendar months don't precisely sync with the moon's orbit. The site reports that it takes the moon 29.5 days to go from full to new to full again. With the exception of February, the months are longer than that, which mean that every so often the timing works out so there are two full moons in one month.
While the term "blue moon" has always typically been associated with an "extra" full moon, the most literal meaning is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere.
Some sources say that the key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre) - and no other sizes present.
Blue moons come along once every 2.7 years on average, and sometimes much more frequently, according to Space.com.
In 1999, for example, blue moons occurred in both January and March (with no full moon in February).
For those residents who live in in the Kamchatka region of the Russian Far East or New Zealand, the blue moon will appear this September. In those locations, Friday's full moon will occur after midnight on Saturday (Sept. 1), making the following full moon - which comes along Sept. 30 - the blue one.