The Moons of Jupiter

As anyone interested in astronomy or its history should know Io, Europa, Ganymede and Calisto are not only the names of four of Zeus’ lovers (or rape victims!) but also the names suggested privately by Kepler and publicly by Simon Marius for the four largest of Jupiter’s moons discovered on 7th and 8th January 1610 respectively by Galileo Galilei and Simon Marius. It must have been an exhilarating experience when they were first observed by those two pioneers of Renaissance telescopic astronomy and it is still an exciting one for an amateur astronomer in the twenty-first-century as related by Clive Thompson in a blog post at The Message. Unfortunately Thompson then goes on to complete misinterpret what that original discovery, four hundred years ago, meant for the cosmology and astronomy of the times. This is a topic I’ve dealt with before but it seems to be one that needs to be addressed at regular intervals like a game of #histsci Whac-A-Mole. What exactly did Thompson say that needs to be banged on the head?

Siderius [sic] Nuncius was a powerful piece of evidence that Copernicus was right: The Earth wasn’t the center of our solar system. The sun was, and the planets revolved around it. Astronomers had been gradually warming up to the idea, and even some church authorities had accepted the Copernican system as a mathematical theory. But by showing that Jupiter had its own moonsthat a planet could be a mini-system of its ownGalileo offered something rather more: Electrifying proof [emphasis in original] of the Copernican idea. You could argue endlessly (and people did) about the geometry and math of various systems explaining how the stars moved through the sky. It was just conjecture.

But proofthat’s different. Once people put their eyes to the telescope and saw those moons circling Jupiter, they had the same whoa-dude reaction that I had on the sidewalks of Brooklyn. The solar system got real. So real, in fact, that the church began to panic; and since Galileo went on to use his telescope to amass even more evidence against geocentrism, including the phases of Venus, religious authorities eventually stepped in and demanded he recant, or else.

To explain what is wrong with the above we first need to know what the accepted view of the cosmos in the first decade of the seventeenth-century. The standard model of the age was an uneasy alliance between Aristotelian cosmology and Ptolemaic astronomy. I say uneasy because the two systems were not actually compatible, something that the scholars of the period knew but chose, mostly, to ignore. It was this geocentric mish-mash that the handful of Copernicans and Tychonians were trying to dethrone. So what exactly was the scientific significance of the Galilei-Marius discovery of the Jupiter moons?

The discovery of the four principal moons of Jupiter didn’t actually have any direct relevance, either positive or negative, for Copernican heliocentricity. What it did do was to refute a central tenet of Aristotelian cosmology that of homo-centricity. Aristotelian cosmology stated that all celestial bodies revolve around the same central point, the earth. The discovery of the moons of Jupiter of course showed this to be totally wrong. Surprisingly this did little or no damage to Ptolemaic astronomy, as this was viewed by strict Aristotelians to already contradict this fundamental principle. In Ptolemaic astronomy the seven planets revolve around the centres of their respective epicycles, which are in turn carried around the earth, actually centred on a point other than the earth, on their deferents. This in the opinion of some Aristotelians was definitely not homo-centricity. This contradiction between the two systems of thought led to various revivals of concentric or homo-centric astronomy over the centuries the most recent being in the sixteenth-century barely a decade earlier than Copernicus’ publication of De revolutionibus. In fact Christoph Clavius, the leading proponent of Ptolemaic astronomy in 1610, regarded the homocentric astronomy of Giovanni Battista Amico and Girolamo Fracastoro to be a greater threat that Copernican heliocentricity and was quite happy to have it shot down by Jupiter’s moons.

Put very bluntly the discovery of the moons of Jupiter by Galileo and Marius was in no way what so ever a proof of the Copernican idea, something of which Galileo was very much aware and he did not try to present it as being one. Marius didn’t even consider it as he was a proponent of the Tychonic system to which he remained true all of his life.

The situation is of course different with the discovery of the phases of Venus. This discovery made independently by Thomas Harriot, Simon Marius, Galileo Galilei and Giovanni Paolo Lembo, the latter a Jesuit astronomer in Rome who probably discovered the phases before Galileo, effectively killed of a pure Ptolemaic astronomy as it proved that Venus, and probably Mercury by analogy (it would be some decades before the phases of Mercury were observed), orbited the sun and not the earth. Once again this is not in anyway a proof of the Copernican system, as there were other competing systems, the Heracleidian, in which Mercury and Jupiter Venus orbit the sun, which, along with the other planets, orbits the earth and the Tychonic in which all the planets except the moon orbit the sun which then orbits the earth, that were conform with the new telescopic discovery. In fact due to the very real unsolved physical problems presented by the concept of a moving earth most astronomers now chose the Tychonic model and not the Copernican one.

Thompson’s final comment about the Church panicking and forcing Galileo to recant is just pure historical hogwash. Any new empirical evidence needs to be confirmed by independent observers. It’s all very well for Professor Galilei the little known mathematicus from Padua to come along and say that he has discovered all of these wonderful things in the heavens with this new fangled device from Holland, if nobody else can see them. What is required is that other independent observers confirm that they too can see all that Signor Galilei claims to have seen. Given the extremely poor quality of the available telescopes and the optical limits of the Dutch or Galilean telescope this was not an easy task. Popular histories criticise contemporaries who failed to see what Galileo had seen but such critics have obviously never tried to observe the moons of Jupiter with a modern Galilean telescope with state-of-the-art good quality lenses, let alone one with very shitty quality seventeenth-century lenses. It is bloody difficult to put it mildly. So who in the end did provide the scientific confirmation that Galileo so desperately needed for his telescopic claims? This confirmation was delivered by the Jesuit professors of the Collegio Romano, the Vatican’s own astronomers. Doesn’t quite fit the picture of a Church in panic, does it?

The true reasons for that oh so notorious trial are far too complex so that I’m not going to deal with them here but I will just say that they have more to do with politics and authority than science. That however is the subject for another blog post on another day.




Filed under History of Astronomy, History of science, Myths of Science, Renaissance Science

24 responses to “The Moons of Jupiter

  1. laura

    It’s a pet peeve of mine, but I always get annoyed by statements like this:

    “You could argue endlessly (and people did) about the geometry and math of various systems explaining how the stars moved through the sky. It was just conjecture,”

    which seem to me to seriously underplay the importance and sophistication of 16-17th century technical astronomy compared to “whoa dude” moments of discovery. Galileo and Marius provided some new phenomena to save, which was important, but it was the increasing accuracy (and physical plausibility) of the mathematical systems designed to save them that allowed people to judge between the systems and that eventually won the day. Science is hard.

    • As I have pointed out on numerous occasions it was principally the accuracy of the Rudolphine Tables that finally persuaded astronomers to accept heliocentricity and not the telescopic discoveries and those are derived from a geometric model, Kepler’s elliptical one.

      • MarylandBill

        It is, in my opinion, a real shame that the Galileo incident focuses so much attention on Galileo. Obviously his astronomical observations were important, but it detracts from the accomplishments that Kepler made which were (IMHO) more significant in the history of astronomy.

      • it was principally the accuracy of the Rudolphine Tables that finally persuaded astronomers to accept heliocentricity

        which still leaves open the question of what persuaded people that the earth actually moves. For of course, you can have a Tychonic version of Kepler’s system, with the planets revolving in elliptical orbits around the sun which revolves a stationary earth. Indeed, Kepler explicitly includes this possibility in the Astronomia Nova, while expressing his personal belief in the fully heliocentric version.

        Bradley’s discovery of the aberration of light? Newton’s Principia? (But as I recall, most of Newton’s rivals, as delineated in E.J.Aiton’s The Vortex Theory of Planetary Motions, either preferred a moving earth, or took a relativistic viewpoint, like Descartes — neither the earth, nor the sun, nor the planets move, relative to the continguous matter of the vortex.)

      • I think that well before Bradley and Newton, you have Jeremiah Horrocks’ determination of the astronomical unit using the transit of Venus in 1639. It wasn’t accurate by modern standards (0.63 a.u.) but even so it would have been sufficient for anyone to realise that the Sun and planets like Jupiter and Saturn were much larger and more massive than the Earth.

      • laura

        Can you have a fully Tychonic version of Kepler’s system? Apologies but I’m genuinely confused about this. Jean-Baptiste Morin published a geocentric version of the Rudolphine Tables, but I’ve never been able to tell from accounts (or my technical astronomy isn’t good enough to understand) if it was a fully Tychonic model or semi-Tychonic with a rotating earth. By 1635 or so, it seems it would have been difficult to save all the phenomena (the transits of Mercury, the motion of sunspots, the eclipses of Jupiter’s moons — all of which became issues only after 1609) without allowing for the diurnal rotation.

      • @Laurence

        I don’t think Horrocks’ dubious attempts to measure the AU played a significant role in the acceptance of heliocentricity. This acceptance had largely been completed by about 1660 and Horrocks’ works first began to be published in 1662.

      • @laura

        It’s geometry vs. physics. In other words, what does it even mean to say the earth “really moves”. Given any motions for the earth, sun, planets, and satellites, you can always refer them to a frame of reference in which the earth is stationary (the so-called ECEF frame, fully Tychonic) or rotates without revolving (the ECI frame, semi-Tychonic). The geometry is identical, but the physics is different.

        In Newtonian mechanics, the earth “really moves” because the neither the ECEF nor the ECI frames are inertial (Newton’s first law holds only with the introduction of pseudo-forces), although the ECI frame is “more inertial”. A related distinction applies in general relativity (locally Minkowskian frames), so Einstein does not retrospectively redeem Aristotle.

        The accuracy of the Rudophine tables has no bearing on this question, at least logically. (I suppose it might have a significant psychological and social effect.) Coriolis-type experiments bear on the question: Guglielmini (1789), Foucault (1851). Bradley’s 1729 discovery of the aberration of light drags optics on stage, but it’s also physics and not just geometry.

        My historical question is, did Bradley’s discovery turn the tide of opinion, or did almost all astronomers already believe in the earth’s motion? If so, why?

      • Coriolis-type experiments bear on the question: Guglielmini (1789), Foucault (1851)

        Riccioli already in the seventeenth-century

      • laura

        @ Michael

        Right, but astronomers always cared about physics to some extent. The most common argument for accepting the diurnal rotation (which e.g. Longomontanus makes in 1622) was that it made no sense for the fixed stars to whirl around the earth every 24 hours, that the “sphere” of the stars would break apart under the strain. As the measurement of solar parallax started to decrease from 3 min (starting with Kepler) and the solar system got bigger, this argument got stronger and stronger. I sort of think of it as the implausible physics of a moving earth slowly becoming less implausible than the physics of a non-rotating one from the perspective of common sense. That’s way before Newton.

        It’s the same with the tables. In the 16th century, the costs in terms of physical plausibility of keeping the earth stationary were fairly small, since you only really had to worry about the positions of the 7 planets. But with all the new phenomena introduced by the telescope, the motions required to explain them all if the earth were to be still (e.g. a third motion for the sun as its axis changes position that Galileo complains about in Dialago) become increasingly implausible (from the perspective of “harmony” at least) relative to the earth having motion.

        Again, I am on thin ice here as my technical astronomy is poor, but it seems to me that most of this complexity can be reduced by allowing for at least diurnal rotation. *My* historical question (or project) is figuring out how many did accept diurnal but not annual motion before 1660.

      • @thony

        Thanks for the link. Unfortunately it comes with Springer’s typical high price tag, but I do have a copy of Graney’s earlier paper on the same topic, “The Coriolis Effect Apparently Described in Giovanni Battista Riccioli’s Arguments Against the Motion of the Earth”. I thought Guglielmini was the first actually to detect the effect. True? The Guglielmini experiments didn’t establish the revolution around the sun, though, just the rotation. So Bradley’s aberration of light might be the first direct experimental confirmation of the annual revolution.


        Well, perhaps. The Longomontanus argument shows that the recognition that the heavens are made of the same “stuff” as the earth, has an impact on the heliocentric-geocentric controversy. So maybe Galileo does get some helio-credit for the Starry Messenger.

        I don’t see why it’s any harder, conceptually, to refer dozens or hundreds of objects to a geostatic frame than just seven. On the other hand, once you’re thinking in terms of actual trajectories (as in Kepler’s famous “pretzel” diagram), it’s pretty clear that you’ll have to go to a fully heliocentric picture to have any hope of simple dynamics. At least it was clear to Kepler; to how many of his contemporaries and immediate successors?

        Why did you pick 1660, by the way?

        Galileo’s complaint about the “third” motion was a typical bit of confusion, or flim-flam (or maybe a bit of both) on Galileo’s part, a failure to fully understand “Galilean” relativity. Although he did have historical precedent if I recall.

        I buy your main argument:

        the implausible physics of a moving earth slowly becoming less implausible than the physics of a non-rotating one from the perspective of common sense. That’s way before Newton.

        if by “common sense” we mean the mind-set of the folks who cared about this stuff. That seems plausible to me. But I wonder how much solid historical evidence we have to back this up.

      • laura

        @ Michael

        We’re probably not much in disagreement. All I would add is that, from my reading, physical arguments (and the idea that the celestial objects were made of the same “stuff” as earth objects) goes back a long way. Regiomontanus makes “physical” arguments though they seem very strange to us, and there’s a huge amount of discussion of what the heavens are made of in the late 16th century (William Donahue has an essay about this subject in Westman’s anthology The Copernican Achievement). Simon Stevin was making highly physical arguments in defense of Copernicus at the same time as Kepler, etc. etc.

        I’m hesitant to try to defend Galileo’s sunspot argument because I’ll probably say something dumb, but as I understand the problem, the apparent motion of sunspots is composed of the sun’s rotation, the earth’s rotation and the earth’s revolution. So to explain the motion of the spots in terms of just the sun’s movements compounds three motions on the sun rather than the usual two before the spots were known. It can be done of course via kinematic equivalence, but it’s a complicated physically implausible motion. Similarly for the moons of Jupiter: their motion, the earth’s rotation and the earth’s revolution, all of which affect how we see the moons, get compounded onto the moons, requiring very strange orbits. If you don’t care at all about physics, then it doesn’t matter. But, like you say, most mid-century astronomers see themselves by this point dealing with actual trajectories of the planets and moons through empty space.

        Though of course I doubt it any of it would have mattered, at least to astronomers, if Kepler’s “physical astronomy” hadn’t produced accurate tables.

        I just picked 1660 because it’s the year most people give for when heliocentric astronomy was accepted widely by non-Catholics. In The Sun in the Church, Heilbron says that Riccioli would have put all the planets around the sun in the Astronomia Reformata (1665) if he hadn’t been bound not to by obedience and I think he’s probably right.

      • @laura
        No, I don’t think we’re in much disagreement at all. Good talking with you.

    • I wonder how many fans of astronomy and even astronomers know that, even well after GG’s time, what has come to be called astronomy was for many centuries previously called natural astronomy?

      That said, I am also wondering what the author of this hugely wonderful blog and perhaps some of his many fans might think of the Mobius Time and Timing pamphlet found at and associated geo-centric clock?

      Warmest regard!!!

  2. You left out a “not” in your last paragraph.

    [Ptolemaic astronomy] in the opinion of some Aristotelians was definitely not homo-centricity.

    True enough, but Ptolemy did show how to reconcile the two systems in his Planetary Hypotheses, which had been given a highly popular exposition by Peuerbach plus Regiomontanus in Theoricae Novae Planetarum. Didn’t you write about that already? Ah yes, The Astronomical Revolution Didn’t Start Here!.

  3. theofloinn

    …the Heracleidian, in which Mercury and Jupiter orbit the sun, which, along with… should be “…Mercury and Venus…”

  4. The idea that Galileo “proved” that Copernicus was “right” (despite being mostly wrong) is not only common, it’s an article of passionate belief for many. Take this guy “Thucydides411”, who insists that the scientific evidence showing that heliocentrism was wholly correct was completely in by 1616 and it was only religious prejudice that kept people from seeing how right Galileo was:

  5. Pingback: Oldest Dutch telescope, archaeological discovery | Dear Kitty. Some blog

  6. Galileo did not get into trouble over theology or astronomy. He just could not help being an asshole, and he paid the price.

  7. the most dangerous aspect of recently discovery of Jupiter moons is Eleven of the moons behave relatively normally, but one beats out of line. The researchers call him a “nerd”.
    want to know about the recently 12 more Jupiter moons discovery? Please visit our site

  8. If you try to use my comments as a pulpit for your own shit, whatever that might be, I will delete your comments and I will block you. Remember, I am the god of this blog!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s