On May 18, a full moon will appear in the night sky. This year, May’s full moon, known as the "full flower moon," will also be—according to one definition—a “blue moon”—a celestial event that happens once every two to three years.

But what is a blue moon? And will May’s full moon be one? There are two definitions of what a blue moon is. Under one, the full flower moon is not a blue moon. Under the other it is.

Neither involves the moon actually turning blue.

We tend to think of full moons occurring once per month. Each month’s full moon is also given a traditional name depending on what was happening at that time of year. For example, May’s full flower moon is named so because it is the time of year when flowers come into bloom, according to the Farmers' Almanac. Next month’s full moon is known as the full strawberry moon because June is the time when strawberries are harvested.

However, sometimes one month has two full moons. This is because the phases of the moon take 29.5 days to complete. This means that there are 354 days for 12 full cycles, so once every two to three years, there is a 13th full moon. Because this moon does not fit into the traditional moon name system of old, it is known as a blue moon.

This is one definition of a blue moon—and one that is technically incorrect.

The other definition is the third full moon in an astronomical season that contains four full moons. Astronomical seasons start and end with spring and fall equinoxes and summer and winter solstices. The spring equinox 2019 started on March 20. This year the spring astronomical season contains four full moons, with May’s full flower moon being the third—hence being a blue moon.

As well as the full moon, May 18 will see a number of other astronomical bodies appearing in the sky. “By the morning of the full moon on May 18, 2019, as morning twilight begins, Jupiter will appear in the south-southwest about 23 degrees above the horizon and Saturn will appear in the south about 30 degrees above the horizon,” NASA said in a statement. “Venus will be rising about 7 minutes after morning twilight begins but should be visible low in the east-northeast until about 30 minutes before sunrise. Mercury will not be visible, lost in the glow of the Sun.”

May 18 will also see a Near Earth Object called 2012 KT12 make its close approach. The object, which measures between 48 and 107 feet in size, is set to pass Earth at 1.0 and 7.5 lunar distances,  and will be traveling at a speed of 8,835 mph, NASA said. A lunar distance is the moon's average distance from Earth.

The next full moon will take place on June 17, at the end of the spring astronomical season. The next season will start on June 21 with the summer solstice.

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