This weekend’s full moon isn’t any old moon, it’ll be a blue moon, according to NASA.
But no, it won’t actually be blue.
Turns out there are two ways to have a blue moon.
The best-known definition of a blue moon is to have two full moons in a calendar month — the second of which is called a blue moon.
According to NASA that happens on average about every 2.5 years.
But another way to define a blue moon is as the third full moon in a season with four full moons. That definition goes back to 1528, and that’s the one being used for this weekend’s full moon.
Astronomical spring (in the Northern Hemisphere) runs from the spring equinox on March 20 to the summer solstice on June 21.
During that time frame there will be four full moons:
1. March 20 (worm moon)
2. April 19 (pink moon)
3. May 18 (flower moon and blue moon)
4. June 17 (strawberry moon)
Since the strawberry moon comes before the solstice that makes four full moons during the spring. That also makes Saturday’s full moon a blue moon.
According to EarthSky astronomers, the last seasonal blue moon was on May 21, 2016.
The moon will hit 100 percent fullness on Saturday afternoon at 4:12 p.m. CDT, according to NASA, but it will appear full from Friday night through the weekend.
May’s full moon also has several other nicknames.
Native Americans used their own names for each month’s full moon, according to the Farmer’s Almanac, which first starting publishing the names in the 1930s.
May’s full moon has also been called the flower moon, the corn planting moon and the milk moon.
The next blue moon will fall on Halloween in 2020, according to the National Weather Service.
There have been moons that have actually appeared blue, but they are rare. One of the better-known examples was in 1883 when the Indonesian volcano Krakatoa erupted, spewing tons of ash high into the atmosphere.
The particles that make up the ash were small enough to scatter the red light and let other colors shine through, according to NASA — making the moon appear blue.
Blue-tinged moons were spotted for years after the eruption.
Blue colored moons have also been spotted more recently after volcanic eruptions — including Mount St. Helens in Washington state in 1980. Forest fires can cause the same effect.
The moon rose at 6:35 p.m. CDT Friday in Birmingham and will set at 5:46 a.m. Saturday. It will rise again at 7:40 p.m. Saturday night and set at 6:27 a.m. Sunday. The last moon of the weekend will rise at 8:42 p.m. Sunday. Click here to see the times in your area.
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