A "once in a blue moon" occurrence will actually happen on Wednesday, and it's set to be an especially rare celestial spectacle: a super blue moon.
Anyone with a clear view of the moon that night will see a slightly brighter and bigger full moon than normal − that's the "super" part of the title.
The "blue" designation doesn't have anything to do with its appearance, however. That just means it's the second full moon in the month.
The two things don't occur simultaneously very often. NASA says it happens about every 10 years, when the second full moon in a month is closet to Earth in its orbit.
NASA says the next super blue moons won't occur for another 14 years, when a pair will grace the night sky in January and March 2037.
The super blue moon will be visible Wednesday, according to the National Oceanic and Atmospheric Administration. At 9:36 p.m. EDT, it will reach its peak.
A blue moon is when a full moon happens twice in a single month, according to NASA.
The moon's average cycle, 29.5 days, is just shorter than the average length of a calendar month. Every two to three years, a full moon will happen in the first few days of a given month; then a second will appear just before the next month begins.
The first full moon in August, which was also a supermoon, was on the first day of the month.
There is also an alternate definition of a blue moon: when there are four full moons in a single season, the third is considered a blue moon, the Farmers' Almanac says.
A supermoon is when the moon is at or near its closest point to Earth in its oval-shaped orbit.
Because of its proximity, the moon will appear brighter and slightly larger, about the size difference between a quarter and nickel, according to NASA.
A supermoon can also cause higher tides than usual.
The upcoming blue moon will be the closest and brightest supermoon of the year, according to Old Farmer’s Almanac.
Contributing: Doyle Rice