There’s supposedly nothing special about a blue moon. A full moon is just a moon phase, and a blue moon is just a designation for certain full moons — the ones timed to fall before the end of a month that already had one full moon.
True, unless it’s like the bizarro blue moon we’re getting in August of 2024, which deserves a fuller explanation.
Why is August’s full moon considered a blue moon?
August’s single full moon gets its “blue” designation from how many full moons fit in a “tropical year.” Typically this means 12, but the current tropical year has 13. And the extra one gets counted as blue. If you find that unsatisfying — perhaps because it’s totally arbitrary and based on concepts like “tropical year” that you didn’t even know existed before today — well, I’m not sure I can help, but I can offer a little more detail.
According to the Library of Congress website, the Maine Farmers’ Almanac’s moon math was derived from that irksome concept I mentioned earlier, the “tropical year,” which runs not Jan. 1 through Dec. 31, but from each winter solstice (which happens around Dec. 21-22) to the next. The year 2024, however, as counted by the “tropical year,” contains four summer moons. The third full moon in a four full moon season — which can’t be the midsummer moon nor late summer moon — becomes a blue moon.
By this system, the summer of 2024 has three full moons with seasonal designations, and a bonus blue moon:
Mashable Light Speed
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June 21 – Early Summer Moon
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July 21 – Midsummer Moon
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August 19 – Blue Moon
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September 17 – Late Summer Moon
Now you know the core logic of what one might call an original blue moon (as if any of this can be considered “logical”).
The moon as captured by NASA’s Lunar Reconnaissance Orbiter.
Credit: NASA / GSFC / Arizona State University
Why are there two types of blue moon?
Actually there are three, because the moon can literally appear blue after a major volcanic eruption, but let’s focus on the ways blue moons are counted on calendars.
The reason you tend not to hear about original blue moons is that the exciting world of full moon tallying was thrown into disarray in 1946 when James Hugh Pruett, writing for Sky & Telescope, fumbled the Almanac‘s definition, incorrectly stating that a blue moon was simply the second full moon in a month. You might call this a “fool’s blue moon,” another term I just coined. James Hugh Pruett’s type of blue moon tabulation caught on, and the blue moon chaos has never dissipated.
So no, nothing about the moon itself is scientifically notable this month. Claiming that certain full moons are “blue” by tradition is a 20th century phenomenon that, according to the Oxford English Dictionary, came along well after the phrase “once in a blue moon.” The moon won’t appear blue (barring any volcanic eruptions), and none of this impacts astronomers, astronauts, lunar beings, or even werewolves. This is, however, a boom time for pedantic trivia junkies.
It’s worth noting that August 2023 also had a blue moon by the other, more popular definition, and two consecutive Augusts, both with blue moons is another rarity, arbitrary or not. According to Space.com, the next blue moon under the more popular blue moon regime will occur in May 2026, while the next original blue moon won’t be for another five years, when it will fall in, yep, August of 2029.
How can I see the blue moon?
Look up.
