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This is really just a curiosity question: I wrote a fantasy novel in which the moon appears blue once every ten years on roughly the same date without fail. If anyone asked me why the moon of my world does this, I would just say “because, magic”, as magic is already heavily present in the story and there are gods that are very ‘present’ (there is no question in the minds of the people whether or not they are merely myth), so handwaving didn’t bother me in the slightest.

However, as I’ve recently become more conscientious about my worldbuilding (though, I don’t mind handwaving if it’s necessary, I just think it’s fun to figure out how certain aspects could’ve realistically come to be and possibly touch on that in the actual story if I want to), I am curious if this would be realistically possible and, if so, how.

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    $\begingroup$ Earthly blue moons are caused by atmospheric conditions, not anything to do with the moon itself. (And forget that nonsense about a "blue moon" being the second full moon in a month, or whatever. It was made up by someone who obviously didn't spend enough time outside at night.) en.wikipedia.org/wiki/Blue_moon#Visibly_blue_moon $\endgroup$ – jamesqf Sep 17 at 5:37
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Biological activity

Some things in nature follow very stable cycles. Cicadas in the US are a good example: depending on the species you will see them once evwry 13 years or once every 17 years.

It may be that your blue moon happens due to such a cycle. Maybe bacteria or some other microbe blooms every ten years.

Solar flares

It may also be that the sun has flare peaks every ten years, and that drives chemical reactions that turn the sieface of the moon blue for a while.

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Comets

There are a lot of ways to measure moon cycles, but this kind of phenomenon belies an explanation on moon rotation, or the angle it hits the atmosphere. Instead, another phenomena may be at work - a comet. Under just the right condition, comet tails can be seen with naked eye - and they can sometimes be blue, like the Comet Holmes. This can easily give the moon the appearance of being blue.

Albeit, there are a few problems. Namely that this requires an orbit atypical to comets - either around the planet or around the Sun such that it would interpose between the planet and it's moon, something I'm pretty sure is unstable. Or it requires different comets every time - which begs the question as to what is causing that. But as far as turning the moon blue, ion trails off a comet will help.

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Something similar to a Blood Moon

Blood Moon is a total lunar eclipse happens when the Moon travels through the Earth's umbra and blocks all direct sunlight from illuminating the Moon's surface. However, some sunlight still reaches the lunar surface indirectly, via the Earth's atmosphere, bathing the Moon in a reddish, yellow, or orange glow.

We can use this phenomenon and combine it with one of the answers posted here that involves comets.

Once every 10 years, a comet passes close to the earth and causes some of its debris to fall into earth's atmosphere, these debris (microscopic dust) acts like a very special prism and remains in the upper atmosphere for quite some time (3-4 days).

During this a lunar eclipse takes place and these special prisms make the light refract to the moon in such a way that only blue light passes through, thus making the moon look BLUE.

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I am curious if this would be realistically possible

Assuming this is a fairly normal moon then yes it's pretty simple.

Blue mould with a 10 year bloom cycle growing on the cheese would give the desired affect.

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