One of the most persistent and pernicious myths in the history of astronomy is that Galileo, with his telescopic observations, proved the validity of the Copernican heliocentric hypothesis and thus all opposition to it from that point on was purely based on ignorance and blind religious prejudice. Strangely, this version of the story is particularly popular amongst gnu atheists. I say strangely because these are just the people who pride themselves on only believing the facts and basing all their judgements on the evidence. Even Galileo knew that the evidence produced by his telescopic observations only disproved some aspects of Aristotelian cosmology and full scale Ptolemaic astronomy but other Tychonic and semi-Tychonic geocentric models still fit the available facts. A well as this the evidence was still a long way from proving the existence of a heliocentric model and many physical aspects spoke strongly against a moving earth. Put another way, the scientific debate on geocentrism versus heliocentrism was still wide open with geocentrism still in the most favourable position.
Apart from the inconclusiveness of the telescopic observations and the problems of the physics of a moving earth there were other astronomical arguments against heliocentricity at the time that remain largely unknown today. Christopher M. Graney has done the history of astronomy community a big service in uncovering those arguments and presenting them in his new book Setting Aside All Authority: Giovanni Battista Riccioli and the Science against Copernicus in the Age of Galileo.
We’ll start with the general summary, as I’ve already stated in an earlier post this is an excellent five star plus book and if you have any interest in this critical period of transition in the history of astronomy then it is quite simply an obligatory text that you must read. So if you follow my advice, what are you getting for your money?
In 1651 the Jesuit astronomer Giovanni Battista Riccioli published his Almagestum Novum or New Almagest , which contains a list of 126 arguments concerning the motion of the earth, i.e. the heliocentric hypothesis, 49 for and 77 against and it is this list that provides the intellectual scaffolding for Graney’s book. Interestingly in discussion on seventeenth-century astronomy Riccioli’s book, and its list, has largely been dismissed or ignored in the past. The prevailing attitudes in the past seem to have been either it’s a book by a Jesuit so it must be religious and thus uninteresting or, as was taught to me, it’s a historical account of pre-Galilean astronomy and thus uninteresting. In fact before Graney and his wife undertook the work this list had never even been translated into English. As to the first objections only a few of Riccioli’s arguments are based on religion and as Graney points out Riccioli does not consider them to be very important compared with the scientific arguments. As to the second argument Riccioli’s account is anything but historical but reflects the real debate over heliocentrism that was taking place in the middle of the seventeenth century.
The strongest scientific argument contra Copernicus, which occupies pride of place in Graney’s book, is the so-called star size argument, which in fact predates both Galileo and the telescope and was first posited by Tycho Brahe. Based on his determination of the visible diameter of a star, Tycho calculated that for the stars to be far enough away so as to display no visible parallax, as required by a Copernican model with a moving earth, then they must be in reality unimaginably gigantic. A single star would have the same diameter as Saturn’s orbit around the sun. These dimensions for the stars didn’t just appear to Tycho to be completely irrational and so unacceptable. In a Tychonic cosmos, however, with its much smaller dimensions the stars would have a much more rational size. Should anyone think that this argument was not taken seriously, much later in the seventeenth century Christiaan Huygens considered the star size problem to be Tycho’s principle argument against Copernicus.
Many, more modern, historians dismissed the star size problem through the mistaken belief that the telescope had solved the problem by showing that stars are mere points of light and Tycho’s determined star diameters were merely an illusion caused by atmospheric refractions. In fact the opposite was true, early telescopes as used by Galileo and Simon Marius, amongst others, showed the stars to have solid disc shaped bodies like the planets and thus confirming Tycho’s calculations. Marius used this fact to argue scientifically for a Tychonic cosmos whilst Galileo tried to dodge the issue. We now know that what those early telescopic astronomers saw was not the bodies of stars but Airy discs an optical artefact caused by diffraction and the narrow aperture of the telescope and so the whole star size argument is in fact bogus. However it was first Edmond Halley at the beginning of the eighteenth century who surmised that these observed discs were in fact not real.
Graney details the whole history of the star size argument from Tycho down to Huygens revealing some interesting aspect along the way. For example the early Copernicans answered Tycho’s objections not with scientific arguments but with religious ones, along the lines of that’s the way God planned it!
Although the star size argument was the strongest scientific argument contra Copernicus it was by no means the only one and Graney gives detailed coverage of the whole range offering arguments and counter arguments, as presented by the participants in the seventeenth-century debate. Of interest particular here is Riccioli’s anticipation of the so-called Coriolis effect, which he failed to detect experimental thus rejecting a moving earth. Far from being a decided issue since 1610 when Galileo published his Sidereus Nuncius heliocentricity remained a scientifically disputed hypothesis for most of the seventeenth century.
Graney’s book is excellently written and clear and easy to understand even for the non-physicists and astronomers. He explains clearly and simply the, sometimes complex, physical and mathematical arguments and it is clear from his writing style that he must be a very good college teacher. The book is well illustrated, has an extensive bibliography and a useful index.
As a bonus the book contains two appendixes. The first is a translation (together with the original Latin text) and technical discussion of Francesco Ingoli’s 1616 Essay to Galileo, a never published but highly important document in the on going discussion on heliocentricity; Ingoli a Catholic cleric argued in favour of the Tychonic system. The second appendix is a translation (together with the original Latin text) and technical discussion of Riccioli’s Reports Regarding His Experiments with Falling Bodies. These experiments are of historical interest as they demonstrate Riccioli’s abilities, as a physicist, as he delivered the first empirical confirmation of Galileo’s laws of fall.
Graney’s book is a first class addition to the literature on the history of astronomy in the seventeenth century and an absolute must read for anyone claiming serious interest in the topic. If you don’t believe me read what Peter Barker, Dennis Danielson and Owen Gingerich, all first class historians of Early Modern astronomy, have to say on the back cover of the book.
 Disclosure; Chris Graney is not only a colleague, but he and his wife, Christina, are also personal friends of mine. Beyond that, Chris has written, at my request, several guest blogs here at the Renaissance Mathematicus, all of which were based on his research for the book. Even more relevant I was, purely by accident I hasten to add, one of those responsible for sending Chris off on the historical trail that led to him writing this book; a fact that is acknowledged on page xiv of the introduction. All of this, of course, disqualifies me as an impartial reviewer of this book but I’m going to review it anyway. Anybody who knows me, knows that I don’t pull punches and when the subject is history of science I don’t do favours for friends. If I thought Chris’ book was not up to par I might refrain from reviewing it and explain to him privately why. If I thought the book was truly bad I would warn him privately and still write a negative review to keep people from wasting their time with it. However, thankfully, none of this is the case, so I could with a clear conscience write the positive review you are reading. If you don’t trust my impartiality, fair enough, read somebody else’s review.
 Christopher M. Graney, Setting Aside All Authority: Giovanni Battista Riccioli and the Science against Copernicus in the Age of Galileo, University of Notre Dame Press; Notre Dame Indiana, 2015