William R. Shea’s research while affiliated with McGill University and other places

Descartes was born in 1596 a full generation later than Galileo and the two men never met. Galileo was seventy-three when Descartes’ first book appeared in 1637 and nowhere in his correspondence does he betray any awareness of the younger Frenchman’s existence even though Mersenne sent him a copy of the Discourse de la méthode. Descartes heard of Galileo, of course, for Galileo’s telescopic discoveries of 1610 created a sensation throughout Europe and were even celebrated in a public lecture at the College of La Flèche when Descartes was a student there. Descartes knew Italian, which was taught to their pupils by the Jesuits, but he does not seem to have read Galileo’s Italian works on hydrostatics, the sunspots and the comets that appeared between 1612 and 1623. Between 1623 and 1625, Descartes made an extended trip throughout Italy but he did not call on Galileo who at the time enjoyed the enviable possible of Mathematician and Philosopher to the Granduke of Tuscany. During that period Descartes was wrestling with problems of mathematics and optics and was only marginally interested in the astronomical phenomena that confronted Galileo.

Everyone is enough of an empiricist to believe that we learn from experience and no one is so far removed from rationalism as to deny that ideas play a vital role in the theories we construct about the world. But it is only too easy, and perhaps too tempting, for philosophers with a pronounced empiricist or rationalist bias to caricature the position of their opponents and make them appear as holding ludicrously simplistic views about the nature of scientific knowledge. In fact, any philosophical position worth its salt has a built-in flexibility or a power to accommodate, sometimes with surprising comfort, theses that seem central to rival theories. The Leaning Tower experiment, however fictitious, is equally well explained by rationalists or empiricists. Facts, however hard or obdurate, have a way of lending themselves to varying interpretations. Properly marshalled, they can be enlisted to serve any good philosophical cause.

We See Through a Glass Darkly Seeing is believing, but we do not see with our eyes only. We look at the world with the aid of inherited images that we may strive to improve but that we do not work to replace unless something dramatic occurs. The tra-ditional Earth-centred system was confirmed not just by the everyday expe-rience of seeing the Sun rise and set, but by the high-powered geometry that was embodied in Ptolemy's Almagest, the result of centuries of diligent observation and detailed computation. Anyone who opens that great classic today cannot fail to be impressed by the mathematical sophistication that is displayed on virtually every page. Better still, those who use the Ptolemaic methods to determine the position of the planets find that they work. Indeed, elementary astronomy is still presented from the standpoint of a motionless Earth, and we learn to calculate where Venus or Mars will be in the night sky on the assumption that the celestial vault revolves once every twenty-four hours. We know, of course, that this is a fiction, but it remains a convenient fiction. It would not merely be pedantic, but foolish, to correct people who say that the Sun moves from east to west by pointing out that it is really the Earth that rotates from west to east. If you doubt this, try playing the rigorous astronomer at the next cocktail party. You may well discover that people are neither impressed nor amused. A more contemporary instance of outmoded representation is our way of conceiving the celestial vault as a two-dimensional surface, a grid on which to plot the position of stars, although we know that there is a third dimen-sion and that the stars are strewn at enormous distances in the vast profun-dity of space. Bigger telescopes and more advanced instruments enable us to plunge ever deeper into the stellar and galactic sea. We reach greater dis-tances because we are carried there by high-tech. If technological development were to come to halt, we could see no further. Our vision is limited by our optics and the reliability of our remote sensors. We could, of course, allow our mind to go on wandering through interstellar space but we would not want to say that a theory, however ingenious, is true unless it is confirmed by evidence. In the absence of telling facts, we can only have suggestive ideas. Current astrophysics is full of clever hypotheses waiting to be winnowed out from the chaff. It is a case of, "Wait and see", or, rather, "Wait until you see". I labour this point because it is essential to our understanding of Galileo's achievement. Copernicus' De Revolutionibus Orbium Caelestium was published in Nuremberg in 1543, nineteen years before Galileo's birth, and he was only two years old when the second edition appeared in Basel in 1566. In other words, by the time Galileo got his first university appoint-ment at the University of Pisa in 1589, Copernicanism was no longer a shocking novelty. Professional astronomers had moved on to Tycho Brahe's compromise system where the planets were made to revolve around the Sun, while the Sun itself continued to wheel around a stationary Earth. The Jesuits, who had the best educational establishments in Catholic Europe, favoured this idea and contributed to its refinement. The Almagestum Novum that was published by one of their professors, Giovan Battista Ric-cioli, in 1651, almost ten ears after Galileo's death, offered a revised version of Tycho's system. It became the most authoritative textbook in astronomy, and was only supplanted by Newton's Philosophiae Naturalis Principia Mathematica in 1687. The most creative and daring Copernican of Galileo's generation was Johann Kepler, Tycho's erstwhile assistant, who placed the Sun squarely at the centre of planetary motions after hundred of pages and several years of calculation. The results of his painstaking efforts were incorporated into a pair of laws that overthrew two fundamental notions of traditional Aris-totelian physics –that celestial objects move in circles, and that those move-ments are uniform. First, Kepler wrote, the orbit of each planet is an ellipse. Second, while travelling along that ellipse, the planet slows down as it moves away from the Sun and speeds up as it nears the Sun. 1 Later, Kepler added a third law: the farther a planet's average distance is from the Sun, the longer it takes to orbit around the Sun; the nearer the shorter. Kepler determined the ratio: the square of the time it takes a planet to complete one orbit around the Sun is proportional to the cube of the average distance of the planet from the Sun.