This weekend's full moon will be the third of four this spring, making it a blue moon.
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An unusual full moon will fill the sky on Saturday night -- a type of moon that won't rise again until 2021.
Many people are familiar with the term "blue moon" and know it as the second full moon in one calendar month. However, there is another way for a full moon to earn this well-known nickname.
"One season -- winter, spring, fall, summer -- typically has three full moons. If a season has four full moons, then the third full moon in the season may be called a blue moon," the Old Farmer's Almanac explained on its website.
This weekend's full moon will be the third of four this spring, making it a blue moon. The first full moon of spring fell on March 20 at 9:43 p.m. EDT, less than four hours after the official start of spring.