Wonderfully detailed discussion of how good science works, warts and all. Exposes the gross misconceptions of science's most famous, revolutionary discovery, even among scientists specializing in astronomy and cosmology. Fabulous addition to the history of the Copernican Revolution, and the historiography of scientific revolutions in general. This makes a great companion to The Copernican Question: Prognostication, Skepticism, and Celestial Order
The popular story of the debate over Copernicanism is usually told as one of religious dogma vs. science. This book presents an opposed viewpoint. In fact, there were serious scientific challenges to the Copernican theory, and in favor of a hybrid geocentric model (Tycho’s), through the time of Galileo.
Christopher Graney focuses his book on Giovanni Battista Riccioli’s New Almagest, published in 1651. The book is in part a defense of Riccioli against misrepresentations of his treatment of Copernicanism, misrepresentations that support the popular story. But Riccioli painstakingly examined the arguments for and against Copericanism, taking as the leading alternative not the Ptolemaic system, but Tycho’s hybrid system, with the sun, moon, and stars orbiting the earth, and the remaining planets orbiting the sun.
The strongest argument against the Copernican system that is discussed by Riccioli is far from a religious argument. It concerns annual parallax and the sizes of stars, and is drawn from Tycho.
Here is an outline of the argument. If Copernicus’s model were true, and the stars sufficiently close, we should be able to measure changes in the positions of stars as seen from the earth at 6 month intervals. When we are on one side of the sun, looking at a star against the background of other stars, its position with respect to those stars (and other points of reference) should differ from its position when we are looking at the same star from the opposite side of the sun. The difference in the star’s position would be a measure of annual parallax, its change in position at opposite sides of the earth’s orbit around the sun.
In fact, measurements taken at the time, including especially those of Tycho, the greatest observational astronomer of the time, revealed no such parallax. Given the lack of measurable annual parallax, the stars must be at a greater distance than previously thought for the Copernican picture to be valid.
Revised distances to the stars wouldn’t make Copernicanism untenable. But many if not most leading observers of the time, including Tycho, Galileo, and Marius, saw, when they looked at the stars, either with the naked eye or with a telescope in the case of Galileo and Marius, a spherical image of discernible size. Estimates of the sizes of stars differed with different observers. But all estimates of the stars’ apparent sizes, coupled with the distance for the stars required to produce no measurable annual parallax, led to a conclusion that the stars must be unbelievably large, on the order of hundreds, thousands, or even millions of times the size of the sun. As astronomer Peter Cruger wrote:
Indeed, if a tiny star had a visible breadth even of a mere 60th of a minute [one second], then according to the above hypothesis its true thickness would be more than nine times that of the Sun, and its volume 730 times greater. I therefore do not understand how the Pythagorean or Copernican Systema Mundi can survive.
In fact, the observations of stars’ sizes were illusory, and some observers suggested so. But no theory of light and optics at the time could explain them as illusory. So, at the time, Tycho’s theory had a distinct advantage in not implying annual parallax, with the earth maintaining a constant position with respect to the orbit of the stars around it.
Another point of contention was what we now call Coriolis effects. The physics of the time did support a notion of “common motion” by which objects on the earth retained the rotational motion of the earth (assuming such a motion) when launched or dropped from objects fixed to the earth. Thus an object dropped from a tower would fall at the foot of the tower rather than some distance from it, as the earth moved between the time it was dropped and the time it landed.
But if in fact the earth does rotate, points on the earth’s surface should rotate at difference speeds depending on their latitude. The effect is now well-known, but at Ricciolli’s time, it was not. In fact, for reasons detailed in Graney’s book, it was not detectable, given the measuring instruments and other limitations of the time. Detectable Coriolis effects would have provided strong evidence for earth’s rotation, but were lacking.
None of this is to say that there were no scientific arguments favoring Copernicanism. Galileo countered some of the anti-Copernican arguments, although the star size problem itself was persistent until it could be demonstrated that apparent stellar diameters were illusory.
Some defenders of Copernicanism themselves appealed to religious argument, Phlips Lansbergen in particular appealing to the majesty of God to argue that the seemingly incredible sizes of the stars following from the anti-Copernican argument were in fact quite acceptable.
Many of the participants in the arguments were of course priests — Ricciolli himself was a Jesuit priest. They could well be motivated by defense of religious dogma, but the weightier arguments themselves are not religious ones.
I’m not an expert and have not read competing arguments against Graney’s. I’ve certainly grumbled at the popular accounts that cast the players in black and white terms, as pure-hearted scientists vs. religious dogmatists. Graney’s book certainly goes a long way to show why that picture may be seriously off-kilter.
It also provides some good fodder for thinking about how scientific theories succeed and fail. Here is a good case where there appear to have been good observational reasons for rejecting a theory that we now fully accept. To suppose that scientific observers and theorists of the time were "wrong" in supporting Tycho's theory is at least a bit ingenuous. By the same token, to suppose those observers and theorists, like Galileo, who supported Copernicanism at the time to have been "wrong" for persisting despite contrary evidence would also be ingenuous.