Posted by: internationalroutier | March 4, 2010

Galileo Was Wrong

many thanks to Captayne Brew for this learned article

Not about everything, obviously.

This month marks the 400th anniversary of the publication of Galileo’s Siderius Nuncius (Starry Messenger), in which Galileo Galilei announced the telescopic observations he had just made of the heavens, following the invention in the Netherlands of the telescope in 1608. Others had made similar observations about the same time, but Galileo was the first to publish, and has gained the lion’s share of the attention since. He claimed that these observations proved the literal, physical, truth of Nikolaus Copernicus’ theory about the nature of the solar system (at that time for practical purposes identical with the “the universe”), and that is how he is generally reported nowadays – as the lone genius who took Copernicus seriously and proved him to be correct against the objections of traditional authority, and especially of the Catholic church. The truth is rather more complex, not to mention a great deal more interesting. There is not space here to set the story out in full, but on this anniversary of the Nuncius I would like to take a brief glance at the proofs that were offered.

Aristotle’s model of the cosmos (based on his own reasoning in the fifth century BC about how things must be) has the sphere of the earth at the centre of the universe, in the position of least dignity. Arrayed about and above it, and concentric with it, are the spheres of the heavens, in which the planets (the moon, Venus, Mercury, the Sun, Mars, Jupiter, and Saturn) revolve uniformly in perfect circles. Beyond the planetary spheres again is the sphere of the fixed stars, and beyond that the prime mover of the universe. The trouble with this model was that it does not fit actual observations of planetary motion. This would not have bothered Aristotle in the least, for he did not regard observation as anything more than a starting point for his thoughts. All the same, the practical deficiency of his model was recognised even in antiquity, and models for the use of astronomers were developed that had the virtue of giving reasonably accurate predictions, even if they bore little resemblance to how Aristotle said things must work. The most sophisticated version was that published by Claudius Ptolemy in the second century AD, and that remained the foundation of practical astronomy for almost another fifteen centuries.

The crux of Copernicus’ theory (already almost seventy years old at the time of the Nuncius’ publication) was the proposal that the centre of rotation in the universe is not the earth, but the sun. That aside, he maintained the rest of Aristotle’s assumptions – concentric crystalline spheres and circular orbits. He produced his own set of predictive geometry to take the place of Ptolemy’s, hoping to reconcile the practical model with the theoretical underpinning in a more elegant whole. Had he succeed it would have been a considerable triumph. As it was his mathematics were about as complex, and with as many special cases, as Ptolemy’s, and the predictive accuracy not much better. In addition, any heliocentric system had a couple of difficulties to overcome. First, it was entirely incompatible with Aristotle’s physics. Although Aristotle had been challenged on many individual points throughout the middle ages, as a comprehensive system his was still the only game in town during Copernicus’ lifetime. Second, a moving Earth implied the testable hypothesis that stellar parallax should be observable, and it simply wasn’t. Again, this requirement (and its non-fulfilment) had been noted in antiquity. The combination of a lack of significant advantage over the current model, a total lack of physical evidence, and the necessity of throwing the whole of established physical theory out the window without even suggesting anything to replace it, made it difficult in the sixteenth century to accept Copernicus’ proposal as anything more than a curiosity.

Galileo was nevertheless an enthusiast. He was determined to find proofs of the theory, and in 1610 he claimed to have done so. Three observations were called as witness.

The surface of the moon was observed to be other than flat. The moon, therefore, was not a perfect sphere. The significance of that was that, according to Aristotle’s cosmology, the orbit of the moon represented the first of the seven heavens. Below that orbit was physis, the changeable and contingent world that we live in. From the moon upward was ouranos, the heavens, which were perfect and unchanging. The heavens were conceived as a realm of perfect forms (spheres) and perfect motions (circles). The evident imperfection of the moon undermined Aristotle’s doctrine of heavenly immutability, but that was hardly news. In 1572 had been observed the first supernova to be recorded in modern times, and many astronomers argued that it must be a sub-lunar phenomenon, since above the moon nothing changes. Tycho Brahe demonstrated, from its lack of parallax, that it must be super-lunar, and therefore that the physical heavens were mutable. This was a heavy blow to Aristotelian cosmology, nearly forty years before the Nuncius. Of course it was also almost thirty years after Copernicus’ death in 1543. Copernicus had cast no doubts on Aristotle’s reasoning concerning the perfection of the heavens, so Galileo’s observation, while it attacked Aristotle, did not support Copernicus.

Four moons around Jupiter were observed, demonstrating that, even if the sun was not the centre of all rotations, the earth couldn’t be either. Again, this challenged Aristotelean cosmology and physics, but did not particularly point to the sun being a centre. If anything it indicated that there was no single centre, and that a new universal theory of gravity was required. Ultimately this was produced by Newton in 1687, but no such theory was available to Copernicus, or even to Galileo.

Individual stars were observed where earlier observers (relying on the naked eye) had seen only nebulous blobs and areas of light.

This lent some support to a conjecture of Copernicus. I have said above that one objection to the earth’s rotating about the sun is that if it is doing so, we should be able to observe stellar parallax. That is, the same star observed six months apart, so that the earth is on opposite sides of the sun, should be observed at a slightly different angle relative to the backdrop of more distant objects. The nearer stars will display a relatively obtuse angle of parallax, and the the more distant a given star is the more acute will be its angle. The minimum distance to the fixed stars (the next thing out beyond Saturn) had been calculated based on planetary distances in the early middle ages. Various figures are given, ranging from 70 to 120 million miles (comparing this figure to the actual orbit of Saturn, it is too low, but only by one order of magnitude). No maximum could be calculated, but while the possibilities were allowed for both that the planetary distances may be greater than the calculated minima, and that the stellar sphere might be very thick indeed, there was no reason to think that the respective distances to the inner surfaces of the spheres of Saturn and of the stars would not be within an order of magnitude or so of each other. In order to make heliocentric theory plausible, Copernicus had to postulate ad hoc that the stars were far further away (by many orders of magnitude) than had been previously thought.

Galileo’s observation of stars that were formerly indistinguishable indicated that some stars, at least, were indeed a very long distance away. This is still not a terribly strong argument, for two reasons. First, Copernicus required not the furthest stars but the nearest ones to be immensely far away. That the sphere of the stars might be of immense thickness was, after all, in conflict with no previous theory. Second, these barely distinguishable points of light might represent not very distant stars, but very small ones. True, it had previously been believed that all stars were individually immensely larger than the earth, but if previous theory had to be overturned, it might as well be the size of the observed object as its distance that turned out to be wrong. It has turned out that the nearest stars are some five orders of magnitude further away than was thought in early modern times – more than enough to defeat naked eye observation, or even 17th century optics. Copernicus’ guess was right, but was entirely unsupported by the available evidence.

At all events, Galileo’s “proofs” turn out to be no such thing. Interesting observations, yes, of which much was to be learned. Aristotle’s credit in the world of natural philosophy took another blow or two, but heliocentrism was not proved, or even much supported. Over the following decades it came nevertheless to be generally accepted de facto. Johannes Kepler’s enunciation in the 1620s of the three laws of planetary motion presumed heliocentric orbits, and their combination of elegantly simple mathematics and precisely accurate prediction was irresistible. Isaac Newton, later in the century, supplied a theoretical foundation for Kepler’s laws. By the eighteenth century for practical purposes geocentrism was dead. With progressive improvements in precision optics the elusive final piece of the puzzle was put in place, proving what nobody had then doubted for over a hundred years. German astronomer Friedrich Bessel first observed stellar parallax, finally proving the heliocentric model, in 1838.



  1. …and today my daughter has a Galileo assignment to do. I shall be very well versed. I also must remember to order a larger brain for when Andy is writing. For astronomy history for all the family (even those of smaller brains) I found helpful.

  2. I trust Erin can be more brief than I was. Glad to be of service – sorry about the brain. I’ll type more slowly in future : )

  3. ’tis entirely the fault of my very small brain. I am endeavouring to grow it biggerer though.

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