Koestler's book presents a rather good history of cosmology from ancient times until the late 17th century. There are four main sections, respectively devoted to the classical world-view (i.e. before the 15th century), Copernicus, Kepler and Galileo, and in each one I was surprised to see just how ignorant I was.
In the first section, I had not appreciated to what extent scientific progress can go backwards as well as forwards. Koestler describes the Pythagorean school - like Penrose, a modern disciple, he considers Pythagoras one of the most important figures in all world history - and shows how they built up a strikingly modern version of astronomy between the 6th and 3rd centuries B.C. Among other triumphs, they correctly deduced that the Earth was round and rotates, and were able to obtain good estimates for its radius, the distance to the Moon, and even the distance to the Sun. Aristarchus, the last major figure in this line of scientists, developed a plausible heliocentric theory and was greatly respected for centuries after his death.
But then Plato and Aristotle severed the link between theory and observation and reverted to a system which placed the Earth back in the middle of the universe, with everything else rotating around it on an increasingly complex system of crystal spheres; this new geocentric theory received its final incarnation in the work of Ptolemy, in the second century A.D. After the fall of Roman civilization, even this was lost, and by the sixth century A.D. the world was flat again. It was interesting to see how it took several hundred more years to rediscover Ptolemaic astronomy, which was then treated (almost literally) as Gospel truth. Koestler makes fun of the medieval mind-set, but I wondered what would happen if our own civilization collapsed and science reverted to a much more primitive stage. Someone who found a miraculously preserved book on General Relativity and mananaged to figure out what it meant probably wouldn't be too critical.
The detailed account of Copernicus was also illuminating, though here, again, I thought Koestler was a little unfair. He paints Copernicus as a timid nerd who was unable to free himself from the Ptolemaic model and strike out in a genuinely new direction, removing the cycles and epicycles altogether. Well, Copernicus could perhaps have achieved more: but I liked the way he patiently worked within the system and showed that, even in its own terms
, it wasn't very good. It isn't as well-known as it should be that the Copernican universe used the same machinery as the Ptolemaic one - intricate arrangements of revolving spheres - but Copernicus's argument was that the arrangement of spheres was simpler if you let the Earth rotate and go round the Sun. It took a while for people to notice Copernicus's work, but when they did the effect was dramatic.
The longest section in the book is about Kepler, clearly Koestler's favorite. I had not appreciated quite how fundamental Kepler's contribution to science was: Koestler argues that he was the first person to formulate a modern scientific law, based on detailed observations and expressed in fully quantitative terms as a mathematical formula, and that he prepared the way for Newton. The process by which Kepler got there is again described in great detail, and I was particularly impressed with Kepler's first attempt to explain the orbit of Mars. His theory was quite good; it agreed with the observations to within 8 minutes of arc, which would have satisfied most people. But Kepler felt he could do better, junked the solution, and spent several more years messing with the data until he derived his First and Second Laws. The accounts of his personal life were also entertaining, and I loved the section about how Tycho Brahe's son-in-law tried to manipulate him into being included as a co-author, but backed down when Kepler added financial conditions to the deal. If he hadn't been so cheap, they would now been called the Kepler-Tengnagel Laws.
But the most surprising part was the chapter on Galileo, which differed from the familiar account to such a large extent that I could hardly believe my eyes. Instead of being a heroic figure cowed into silence by the reactionary forces of the Inquisition, Koestler's Galileo comes across as an arrogant and dishonest jerk. The disagreement with the Church is usually portrayed as simply being about the question of whether the Earth went round the Sun or vice versa, with Galileo clearly being the good guy. Koestler points out a host of perplexing divergences from the myth.
To start with, Galileo was not defending state-of-the-art science, which was Kepler's system, but the outdated Copernican universe, by then nearly a century old; he had never bothered to read Kepler's books properly. The contrast was not against the traditional Ptolemaic system (everything goes round the Earth), but against the much more sophisticated system proposed by Tycho Brahe (the Sun and the Moon go round the Earth, all the other planets go round the Sun). And worst, Galileo had in fact no evidence at all to support the Copernican system against the Tychonian! The only thing that would have helped him was evidence that the stars moved slightly every year as a result of the Earth's movement around the Sun; but his instruments were nowhere near sensitive enough to measure stellar parallax, and in the event he cheated and fabricated a transparently incorrect argument. I had seen a related version before in Feyerabend's Against Method
, but wasn't sure I should believe it. Well, clearly I must check this with the primary sources, which I am ashamed to say I have not read.
Koestler's book is by no means perfect. He puts in more detail than he needs to, sometimes for no obvious reason, and it feels too long. He is not very good at science, and it is painfully clear why he stopped with Newton: he doesn't even seem to understand Newton's theories properly, much less the 20th century ones that he sometimes brings in as comparison points. I found most of his digressions into philosophy unconvincing. But he's found an astonishing amount of good material and assembled it into a compelling story. If you're interested in learning where modern science comes from, you might want to check him out.