“…realigning the heavens with a single stroke of the brush.“ – Really?

Recently on twitter I stumbled across a problematic discussion, as to which single image had most changed the course of science. Although the various participants made stimulating and interesting suggestions, Darwin’s tree diagram, Franklin’s photo of DNA etc. I found this discussion problematic because it suffers from the same difficulties as discussion in the history of science as “the first”, “the greatest”, “the father of” and all similar hyperbolic claims, just how do you measure and compare the numerous candidates that spring to mind?

This discussion didn’t just appear out of cyberspace on somebody’s whim but was provoked by Joe Hanson at It’s OK to be Smart and his post Message from the Moon, which in turn was provoked by the set of washes of the moon by Galileo that had been circulating on Twitter a couple of days before.

Galileo's washes of the moon.

The watercolour sketches that Galileo made of his initial telescopic observations of the moon in 1609/10 are iconic images in the history of science that did have a major impact on the way humanity viewed the cosmos but there are an awful lot of inaccuracies in Hanson’s description of that impact that I am going to analyse here.

Hanson’s first minor error is to claim that the images he has posted on his blog are included in the Sidereus Nuncius. Galileo’s legendary publication does indeed included woodcuts of five of his lunar watercolours but the sheet displayed by Hanson, and here above, was not included, a trivial but important point.

Hanson informs us:

But hiding in their shadows lies a greater significance. The squiggled edges of that bleeding ink bear an observation that altered the heavens themselves. Or at the very least, our view of them.

And then goes on to explain why:

In 1610, cosmology, not that it had much to show for itself as a science, was still based on the ideas of Aristotle, who by this time had been dead for 18 centuries. So current! Copernicus’ observation that the Earth orbited the sun, first published in 1543, had begun to challenge Aristotelian supremacy, it wasn’t exactly a popular idea. 

Aristotle’s cosmological beliefs were based on the idea that the heavens were made of a perfect substance called “aether”, and therefore the circular motions and spherical shapes of heavenly bodies were also perfect. Earth, he claimed, was inherently imperfect, as were all the things that existed upon it. Everything in the heavens was awesome, and Earthly matter was inherently “just okay”, even if its name was Aristotle. This was one of the reasons people found Copernicus’ claims so hard to swallow. The imperfect Earth among the perfect heavens? Heresy! [emphasis in original]

Somewhat sloppily expressed but so far so good, although placing the earth in the heavens didn’t really play that much of a role in the initial rejection of Copernican cosmology being insignificant in comparison to the physical problems engendered by a moving earth. Hanson’s argument is that because Galileo’s interpretations of what he saw through his telescope, and don’t forget that they are interpretations, clearly suggested that the moon was not smooth and perfect but had a landscape like the earth he had realigned “the heavens with a single stroke of the brush”; a nice literary figure of speech but unfortunately one that doesn’t fit the historical facts.

Already in antiquity people, had speculated that the differing shades of the moons surface were the result of a mountainous landscape. This viewpoint was expressed most notably by Plutarch in his On The Face Which Appears in the Orb of the Moon, one of his collection of essays, the Moralia. This was well known and widely read in the sixteenth-century and was even used by Kepler as a springboard for his own “lunar geography”, the Somnium, written but not published before Galileo made his telescopic discoveries. This widespread alternative concept of the lunar surface made it much easier to accept Galileo’s discovery and considerably weakened any impact that it might have had on Aristotelian cosmology. However this was not the only factor that gives the lie to Hanson’s “single stroke of the brush” postulate. Aristotle’s division of the cosmos into two spheres one superlunar, which was perfect, unchanging and eternal, everything below, and the other sublunar, which was imperfect, constantly changing and subject to decay had been under attack for most of the century preceding Galileo’s discoveries, as I have already outlined in my post on Comets and Heliocentricity.

In the 1530s observations of several comets had led many leading European astronomers to the conclusion that comets were superlunar phenomena and not sublunar ones as Aristotle’s cosmology required. Comets are of course anything but perfect, unchanging and eternal. In the 1570s another generation of European astronomers, Tycho Brahe and Michael Maestlin to the fore, confirmed this conclusion making life more than somewhat difficult for any cosmologist who wished to maintain a strict Aristotelian party line. To make matters worse the stellar novae of 1572 and 1604 observed once again by Europe’s finest watchers of the heavens and determined by them to be unquestionably superlunar really put the kibosh on Aristotle’s wonderful division of the cosmos. All in all by 1610 Aristotle’s cosmology was already looking distinctly unhealthy and Galileo’s discovery of the lunar landscape far from being an unexpected deadly bolt out of the blue was just another blow helping it on its way to its grave.

Hanson might be forgiven for his over emphasis of the impact of Galileo’s lunar watercolours based obviously on his ignorance of Renaissance astronomical and cosmological history but the content of his closing paragraph displays an ignorance that I, for one, find hard to forgive. Our intrepid non-historian writes:

Galileo’s Sidereus Nuncius [emphasis in original] also included newly detailed maps of the constellations and the mention of four moons of Jupiter (although detailed observations of those were still centuries away), [my emphasis] but it was his drawings of our moon that bore the most impact on future astronomical science, realigning the heavens with a single stroke of the brush.

Having over emphasised the significance of the impact of Galileo’s lunar watercolours Hanson dismisses his discovery of the moons of Jupiter in a throwaway comment. He couldn’t demonstrate his ignorance of the material more spectacularly.

It was of course Galileo’s discovery of the four largest moons of Jupiter that caused the sensation and also did the most damage to Aristotelian cosmology, when he published the Sidereus Nuncius in 1610. Central to Aristotelian cosmology was the principle of homo-centricity, i.e. the concept that all celestial bodies, the sphere of the fixed stars and the seven planets, revolve around a common centre, the earth. The discovery of the Galilean moons, as they came to be known, was a direct empirical proof that the principle of homo-centricity was wrong. It lent indirect support to heliocentricity, which required two centres of revolution the sun for the fixed stars and the six planets and the earth for the moon. It was Galileo’s discovery of the Medician Stars, as he called them, which led to his much desired appointment as court philosophicus and mathematicus in Florence and professor of mathematics at the University of Pisa without teaching duties. Catapulting him almost overnight from being an obscure, ageing professor of mathematics to being Europe’s most notorious astronomer. The four moons of Jupiter are not “mentioned” in Sidereus Nuncius they are the reason for its hurried and secretive, to prevent anybody else beating him to the punch, composition and publication.

The illustrations of the moon in the Sidereus Nuncius are the eye candy, which the reader can admire but the far less visually spectacular diagrams of the positions of the four moons relative to Jupiter are the explosive content that make this slim pamphlet one of the most important scientific publications of all time and elevated Galileo into the pantheon of scientific heroes.

Page from Galileo's observation log displaying position of the moons relative to Jupiter

Page from Galileo’s observation log displaying position of the moons relative to Jupiter


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

17 responses to ““…realigning the heavens with a single stroke of the brush.“ – Really?

  1. Recently on twitter I stumbled across a problematic discussion, as to which single image had most changed the course of science.

    That’s a discussion topic that calls for hype and subjective personal opinion. So why are you surprised that it led to hype and subjective personal opinion?

  2. laura

    I’m not sure I totally agree with this. In terms of culture, Galileo’s account of the moon’s surface had a huge impact — from my reading it’s almost always the earth-like moon that shows up in popular 17th century presentations (e.g. Ben Jonson’s The Staple of News) — though like you say probably because it was already an idea people were primed to accept.

    In the Cappellan or Tychonic schemes, which were very intellectually creditable at the end of the 16th century, Venus and Mercury (at least) were like moons of the sun, so I’m not sure why Jupiter’s moons would be such a big deal conceptually. I know the degree of acceptance of “new” astronomy differed between Italy and northern Europe in 1609 and the impact varied from place to place, but it’s always struck me (well, since I started thinking about this stuff) that the importance moons of Jupiter is overstated and the importance of the rocky possibly-inhabited moon understated in the general Galileo story.

    • Marjorie Hope Nicolson would certainly argue that the earth-like moon in popular 17th century presentations owes more to Kepler’s Somnium than to Galileo’s Sidereus Nuncius. Although both certainly played a role.

      It is almost impossible to overstate the impact of the discovery of the moons of Jupiter on the development of astronomy in the 17th century.

      • In my post I wasn’t trying to deny that Galileo’s moon watercolours had an impact, they most certainly did, but to point out that they were by no means solely responsible for refuting the Aristotelian concept of a perfect unchanging heaven, as Hanson was claiming.

  3. I wonder what Hanson regards as “detailed observations” of the moons of Jupiter? The paper by Débarbat and Wilson describes frequent observation by plenty of astronomers in the decades immediately following the discovery of the satellites.

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  6. Just ran across the New Yorker article A Very Rare Book (free access until September), about a notable recent forgery involving the Siderius Nuncius, the moon watercolors, and the whole antique book trade, but especially Renaissance astronomical works. The history of science is only glancingly referenced, but I think most readers of this blog will still find the article well worth their time.

    • Baerista

      A better (imho) and more up-to-date account of events is found in the current issue of JHA: http://jha.sagepub.com/content/current

      • Thanks for the tip. An excellent article by Al van Helden, a man who really knows his stuff.

      • The same issue of JHA also contains an intriguing article by Nicholas Jardine, “Kepler = Koestler: On Empathy and Genre in the History of the Sciences”.

      • An excellent article by Al van Helden, a man who really knows his stuff.

        Yes indeed. His article “Galileo, telescopic astronomy, and the Copernican system” makes a nice complement to your Transition series. Sample quote: after outlining the tripartite choice (Ptolemy, Copernicus, Tycho), van Helden writes:

        This dramatic chapter in the history of astronomy was dominated by Galileo Galilei (1564–1642), no mathematical astronomer but a brilliant observer and thinker, who became a passionate advocate of the heliocentric world picture.

      • Galileo’s role was nowhere near as central and dominant as Al would like us to believe. I’ve argued personally with him about this but he’s not prepared to modify his standpoint.

      • van Helden also writes, in the same article:

        Italy was the only Catholic region in the forefront of science where cosmological speculation was severely hampered by the Galileo affair. In Catholic France the intellectual climate was considerably less restricted.

        and then goes on to cite Gassendi and Boulliau as Catholic clerics who “published their Copernican astronomical works without serious interference.”

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  9. Reblogged this on συμποσίον ἀκταῖος κατακηλέω and commented:
    Images that change the course of history?

    “Here too it’s masquerade, I find:
    As everywhere, the dance of mind.
    I grasped a lovely masked procession,
    And caught things from a horror show…
    I’d gladly settle for a false impression,
    If it would last a little longer, though.”
    ― Johann Wolfgang von Goethe

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