A "blue moon" will appear in the night sky on May 18. This relatively rare celestial event, involving the third full moon in an astronomical season with four full moons, will not happen again until August 2021.
Viewers wishing to watch the blue moon live online can do so below. The live stream is provided by the Virtual Telescope Project, which is part of Italy's Bellatrix Astronomical Observatory. The facility offers real-time observations with remotely controlled telescopes. During the program, which will begin at 1:45 a.m. EDT, cameras will show the moon rising above Rome's skyline.
There are two prevailing definitions of a blue moon. One—incorrect—says it occurs when there are two full moons in one month—the second of the two is sometimes called a blue moon. But as the U.K.'s Royal Museums Greenwich noted, this interpretation arose in the 1940s and was "perpetuated by radio shows and the Trivial Pursuit board game through the 1980s."
The correct definition of a blue moon—or an "astronomical full moon"—says it occurs when an astronomical season has four full moons.
Astronomical seasons begin and end with the equinoxes and solstices. Spring began with the equinox on March 20 and will end with the summer solstice on June 21. Normally, there are three full moons per astronomical season. However, because it takes about 29.5 days to complete a full lunar cycle, occasionally there are extra full moons.
A blue moon is the third full moon of an astronomical season that has four full moons.
The last astronomical blue moon took place on May 21, 2016, and the next one will be on August 22, 2021.
May's full moon is also known as a flower moon, which relates to the traditional names once given to full moons by early cultures in Europe and America. It's called a flower moon because May is the time of year when flowers bloom.
Despite the name, a blue moon does not mean the moon turns blue. "Blue moons remain the same color as any other full moon except in two rare cases. During a lunar eclipse, the moon can turn blood red, lit only by the light that is bent around Earth by its atmosphere onto the face of the moon. As lunar eclipses occur only during full moons, and blue moons are one type of full moon, very rarely a blue moon will be red," Royal Museums Greenwich said in a statement.
"In very rare circumstances, the moon can appear blue, but in this case it is a color added to the moon by viewing it through a haze of dust particles in our atmosphere, perhaps from a recent volcanic eruption. In this case, from space, the moon will look just as gray as it always has."
