Does Jupiter have moons in Mythic Europe?

So, I was brushing up on my 16th century astronomy, which lead me to reading the evidence against the geocentric model of the universe - which was, at the time (before Kepler took Tycho Brahe's data and built up his 3 laws of Planetary Motion) was:

  1. Venus goes through all 4 phases, which can't happen in the geocentric model of the universe, and

  2. Jupiter has observable moons, which means that not everything intrinsicly revolves around the Earth.

I'm guessing #1 won't occur in Mythic Europe, as the geocentric model is objectively the case. But is that the case for Jupiter's moons? This is more a question about the geocentric model, I suppose - does it REQUIRE that everything revolve around the earth, or is it possible for planets to have things that orbit them, as well?

There could be.

Start with the basic facts:

planets move around the earth in circles (because circles are perfect)

Alas that doesn't seem to fit with observations.

So they must move on circles which move on circles. These are called epicycles, and there was an infinite staircase of these to get the actual perceived motion.

Someone seeing the moons could interpret them as manifestation of the epicycles?


I think that, if you really wanted, you could definitely have moons orbiting jupiter. This is particularly true if your saga revolves around some new discovery that changes the paradigm of the time.

However, I also think that it would be more fitting if no celestial body (other than Earth) had "moons"; both in terms of astronomical understanding of the time, and in terms of the philosophic/magical implications.

  1. I believe the phases are OK in the geocentric model as Venus is closer than the sun, just like the moon. However, the location of Venus in the sky combined with those phases is a problem.

  2. Who (ignoring possible Chinese claims as they're not in Europe) observed the moons of Jupiter prior to Galileo about 400 years after canon? Was this really an issue in the 16th century, not the 17th century? If you're ignoring #1, is this really a problem?

Do you need it to?

I'd argue that no, it doesn't, but that shouldn't stop you having them if you want them.

Seeing as Jupiter's moons aren't visible to the naked eye, their existence won't come up unless your characters deliberately start experimenting with Imaginem magic to see what you can do with species originating from a very long distance away, or unless you attempt travel far beyond the lunar sphere.

If you want them to, then there's no reason there can't be strange bodies on the celestial sphere that Jupiter lies on, that move in a pattern around Jupiter. Are these moons? Are these the reflection of powerful celestial spirits? Who knows, and indeed it would take a colossal amount of research to find out the answer. Maybe an astrological mystery cult already has an answer, but won't share it.

It's actually the "all phases of Venus" issue - the geocentric model predicts that Venus would only have crescent or new phases. In fact, it also has full and gibbous. (Full, in particular, means that it's on the opposite side of the earth compared to where the sun is.) Here's a fun little animation that describes the issue:

Gah, you're right- Galileo was doing his thing in the early 1600's (1610, publishing in 1613).

Again - this is a question mainly about the historical Ptolemic model - if there was anything that expressly prohibited things other than the Earth having their own moons and whatnot. Historically, the phases of Venus was considered definitive proof, but the moons of Jupiter were (I think) merely 'supplementary'. I was wondering if anyone knew just how supplementary they were.

I see where I was off. I was thinking of the Eudoxan–Aristotelian model as opposed to the deferent and epicycle model, and I should really be thinking of the latter - I'm just not familiar enough with all the variations. In the former Venus has all phases, but it shouldn't only be seen close to the sun. In the latter these two switch.

However, again, who saw phases of Venus prior to Galileo (again in the early 17th century)? Was this really an issue in the 16th century, let alone close to 1220?

Well, with epicycles you do have planets orbiting points that are orbiting Earth. Once you've gotten there, you're basically at the behavior of moons and planets, though still stuck with the geocentric model. So I don't see why there couldn't be objects whose epicycles are around other planets. Maybe some philosopher considered this at some point?

Is there any reason to assume - or for that matter care - whether any particular model, Ptolemaic or otherwise, is correct? Hermetics are bound by the Limit of the Lunar Sphere and I can't think of any other magical tradition that can surpass this either. If magi develop some sort of equivalent to telescopes, let them be as surprised and puzzled as a real-world observer would be. Nailing everything down in advance just kills stories.

Having some sort of vague plan helps avoid plot holes.

Yeah, when I first tried visualizing this, I kept on thinking "no, Venus can get phases just like the moon! This makes no sense!" - but the animation, and the idea that Venus was moving in a similar orbit as the Sun (and thus would always appear near it) made it clear to me.

According to the wiki ( ... servations), anyone with really good eyesight and good atmospheric conditions can see the phases of Venus. (Ie, Per 5, or folks with Per 2+ Keen Vision). So, it's entirely possible that someone in Mythic Europe has actually seen this. However, as the geocentric model is objectively true, they haven't seen the full 4 phases.