This weekend's full moon is a rare 'seasonal blue moon': What that means

The big glowing globe in the night sky on Aug. 22 will be a very specific kind of blue moon.

Amanda Kooser
Amanda Kooser
Freelance writer Amanda C. Kooser covers gadgets and tech news with a twist for CNET. When not wallowing in weird gear and iPad apps for cats, she can be found tinkering with her 1956 DeSoto.
2 min read

A brilliant full moon rises at NASA's Kennedy Space Center in Florida in 2017.

NASA/Kim Shiflett

Once every 2.5 to three years, according to NASA, we get a "seasonal blue moon." That's what we're getting with the full moon coming up this weekend. So how is a seasonal blue moon different from a regular blue moon? Let's sort this out.

The term "blue moon" has become popular as the reference to a second full moon that happens within the same month. We had a lovely one last year on Halloween. But our August moon this year is the only full moon this month. 

NASA gave a history lesson on blue moons, dating the term to at least 1528. "As the third full moon in a season that has four full moons, this will be a blue moon by the older definition," NASA's Gordon Johnston wrote in a full-moon guide for August and September. The use of "blue moon" for the second in a month traces back to the 1940s. 

The term "seasonal blue moon" can help differentiate between the older and more modern definitions. Unfortunately, blue moons aren't actually blue-hued.  

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The August moon will be full on Sunday morning at 5:02 a.m. PT, but it will appear full from Friday through Monday morning, so you can enjoy the night-light glow across the entire weekend. Timeanddate.com can give you the moon rise and set times for your location.

If you want to call the August full moon by its nickname, you can refer to it as the sturgeon moon, after the fish. So that will make it a full seasonal sturgeon blue moon, if you want to make a celestial word salad out of it. All that matters is that you step out and enjoy the view of our natural satellite lighting up the night.

Follow CNET's 2021 Space Calendar to stay up to date with all the latest space news this year. You can even add it to your own Google Calendar.