Blue Moon alert.

[Mark Wagner]

Early morning next Sunday is a Blue Moon.  Interestingly, there exists more than one definition of this rarity.  How rare? A Blue Moon usually occurs every 2-1/2 to 3 years.

I recall witnessing two in a year!  That was 2018.  When's the next "Two Blue"?  2037!

[Peter Natscher]

https://www.youtube.com/watch?v=v0fy1HeJv80

[Mark Wagner]

The Ultimate Blue Moon to my mind, since it means 13 full moons  in a year, would be a blue moon in Ophiuchus since it is thought of as the 13th Zodiacal constellation.  Some sources (ChatGPT) say this Sunday will actually be that (Scorpius has the least moon transit).

Peter, I recall that song featured in the movie American Werewolf in London.  And the great Warren Zevon song Werewolves Of London.  All, of course. Having to do with full moon.

[Brad Templeton]

Not a blue moon, alas.      The idea that a blue moon was the 2nd full moon in a month was apparently due to a mistake by a writer in Sky and Telescope (of all publications) a long time ago, but in the 20th century.    The writer misread a different rule (4th full moon in a season) and expressed it as 2nd in a month, and this got repeated in the popular press.

So in effect, it's a 20th century version of one of those Facebook memes that people keep forwarding that were based on a mistake or fabrication.

Not that the rule of 4th full moon in a season was anything real, who knows how the writer of that came up with it, but it dates to much further back.

Historically it was just a metaphor for a very rare event, since the moon is not usually blue, though there have been atmospheric pollution events that made that happen.   It didn't really have anything to do with the moon, and I am guessing the almanac one was just somebody trying to make it have something to do with the moon retroactively.

And *that* sort of retroactive definition game happens quite often, not just once in a blue moon.

On Monday, May 25, 2026 at 7:44:47 AM UTC-7 Mark Wagner wrote:

[Mark Wagner]

ChatGPT gives a different origin but cites S&T later:

"In 1937, the Maine Farmers’ Almanac used “Blue Moon” for a special seasonal full moon: specifically the third full moon in a season containing four full moons."

It also states two versions (including  two in a month) are in current use nonetheless.

Every time I find something I loved is not as it seemed, I get blue, but only if it's the second time in a month.  

[Brad Templeton]

No, that's what I read though I didn't catch about it being the 3rd of 4, rather than the 4th.    3rd of 4 doesn't make much sense to me, but who knows.    In any event, both that, and the S&T misquote of it, are both retroactive explanations of an old phrase that originally did not have anything to do with the actual moon.  Social media didn't invent the idea of the made up meme.

[Mark Wagner]

This might be useful.  Library Of Congress article on blue moons. 

Explains seasonal blue moons, the tired of four question, and the "modern" blue moon meaning two full in one month.  Sunday is a "modern" blue moon.

https://share.google/j1sjIhfPC4ZsKnbun

It does not address actually blue moons, which are atmospheric, resulting from dust and volcanic activity.

[Brad Templeton]

I will continue to be a stickler on this one.   While there is not an authoritative source, all of these interpretations strike me as retroactive.     That doesn't mean they have not become widely known, even the one that everybody agrees started as a mistake.

When I say they are retroactive, I mean that the phrase "once in a blue moon" goes back to the 16th century.   And for 300 years it did not refer to any lunar phenomenon.     It meant something extremely rare (or that never happens.)

Then various people decided they should like to make up a story to describe some sort of rare, or even non particularly rare lunar phenomenon.     That includes the volcanic dust making the moon appear blue.   That is not a blue moon, that is something somebody heard of and said, "let's call that a blue moon."   Same for the other instances with the 3rd of 4 full moons, or the 2nd in a month.   The term came first, and the explanation was  a retcon.      The original reason that a blue moon meant a rare event is lost, or it simply was because the moon is generally not blue.  These other meanings conjure up a false story of somebody seeing a rare event and saying, "This only happens as often as 4 full moons in a season" or "This only happens as often as air pollution turning the moon blue."     None of those happened.

However, if you want to pretend the retcons are real, you may. But know they are retcons.      One can announce that there is a blue moon at the end of this month, but that is not a statement that has any meaning or denotes anything of astronomical significance.    Some of the other moons do have some meaning.   The harvest moon gave some extra light to work on the harvest.   Ditto Hunter's Moon.  The others that have names are generally quite obscure.   Strangely, of late I have been getting annoyed by certain press getting overexcited as all sorts of astronomical events  of various provenance.     Supermoons, micromoons, named full moons, minor meteor showers, non-close planetary alignments, very dim comets, etc.      And perhaps the most bothersome are the blue moons.

Now I love astronomy, so why am I bothered at the mainstream press shouting the praises of the sky?   Because I think hype, in the end, just does more harm than good, and leads to disappointment.    Telling people to go out to watch a meteor shower with a ZHR of 20 (and that's only after local midnight in dark skies) just is going to lead them to disappointment.   Saying to watch the "supermoon" which is 1% bigger than average and you couldn't possibly notice it, is again going to just disappoint.    All these things will disappoint.  The blue moon is the most nothing of all; just an invention about a modest coincidence of 12-month and lunar calendar.

Promote the cool stuff, the eclipses (even partial though make it clear how very different those are from total,) and naked eye comets, and even this year's new moon Perseids (though even they aren't that spectacular unless you get people out after 1am in dark skies.)  A Bortle 3 milky way.   Close alignments of planets you can actually notice.  Occultations of planets or bright stars.   Transits of the sun.   Things people can actually be impressed by.   Plus of course views of great objects in telescopes and binos.  These are the things that might recruit a new astronomer.

[Mark Wagner]

Be a stickler.

But I bet people will continue to use the term,  regardless.

I like the moon, red, white or blue.  But I like it new best.