The Galileo Affair - (1) The Problem with Heliocentrism

At rest, however, in the middle of everything is the sun. For in this most beautiful temple, who would place this lamp in another or better position than that from which it can light up the whole thing at the same time? For, the sun is not inappropriately called by some people the lantern of the universe, its mind by others, and its ruler by still others. [Hermes] the Thrice Greatest labels it a visible god, and Sophocles' Electra, the all-seeing.

Nicolaus Copernicus

The most wretched and frail of all creatures is man and withal the proudest. he feels and sees himself lodged here in the dirt and filth of the world, nailed and riveted to the worst and deadest part of the universe, in the lowest story of the house, the most remote from the heavenly arch"

Michel Eyquem de Montaigne

Heliocentrism; It’s pretty obvious isn’t it?. How could anyone have been stupid enough to believe that the earth is the centre of the universe?; it must have been because they were a bunch of ‘sky fairy’ worshipping simpletons, whereas you are a modern, enlightened man of science and reason. Unfortunately what you have just demonstrated is the mode of thought known as ‘patronising historical hindsight’; but don’t worry, you are not in the minority, in fact you find yourself in pretty illustrious company. It is in the nature of man to denigrate his forebears. It could not be otherwise. However, for an honest assessment of the Galileo affair we need to get back into the minds of the protagonists and see things as they saw them. In all likelihood, had you been alive at the time of Copernicus, you would also have rejected the Heliocentric model as an interesting but silly mathematical fiction, for the simple reason that this was the opinion of the overwhelming majority of astronomers at the time.

The traditional geocentric model had been held by astronomers since the very beginning of the discipline. One of the few exceptions to this was Aristarchus of Samos whose work is referenced in Archimedes’s book 'The Sand Reckoner'. His model was thought ridiculous and largely ignored for reasons we will come to. In fact, as Plutarch notes through a character in one of his dialogues, it was so unpopular that:

Cleanthes (a contemporary of Aristarchus and head of the Stoics) thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe (i.e. the earth), . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis.

And so Aristarchus’s theory was ignored, until in 1543 an obscure Catholic Church administrator by the name of Nicholas Copernicus published a book called ‘De revolutionibus’ in which he defended the heliocentric alternative as true. In his work he created detailed mathematical models which allowed him to predict planetary observations with a high degree of accuracy. Since his calculations worked pretty well, why did almost all of his contemporaries and immediate successors reject the proposal?. The answer is that the evidence in favour of it was indecisive, one might almost say weak.

The sole advantages of the model were in the elusive realms of beauty, theoretical elegance and intelligibility. For example, it eliminated all non uniform motion from Ptolemy’s planetary model by getting rid of the equant, and all motions in Copernicus’s model therefore follow the Aristotelian notion of uniform circular motion. It had important explanatory possibilities; it explained the periodic reversal of the planets against the background of the stars as an optical illusion thus bringing intelligibility to a previous inexplicable feature of the universe. It also explained why mercury and Venus are never seen very far from the sun (they are in small orbits).

Against these there were amassed a series of decisive disadvantages. There was no observation capable of proving the earth was a planet. Predictions using the system were no more accurate than that of the ancient Ptolemaic system. Heliocentrism represented a massive violation of common sense. It destroyed the only coherent system of physics available; the Aristotelian system. Aristotle’s world was built on a stationary earth and a rotating and revolving Earth was deemed an absurdity because the motions would require very large speeds. If this really was the case, how was the earth supposed to be moving?. Where was the physics to explain this movement?.

Hurling the earth into the heavens also destroyed the dichotomy between heavens and earth that had previously been central to European thought for 2000 years. According to the accepted cosmology of the period our miserable sphere was located at the bottom of the celestial hierarchy, considered too unworthy to be part of the heavens due to its imperfect and sinful nature and with hell and purgatory placed at its core. Our planet stood in dismal contrast to the heavenly firmament above, a realm of perfection derived from Plato's Theory of Forms with the realm of God beyond. Out of all celestial bodies our earth was emphatically the Detroit of the Cosmos. Promoting it to the heavens required a vast conceptual leap.

Another question the model raised is if the sun is at the centre of the universe, why doesn’t everything fall into it?. Copernicus’s explanation for this was that ‘earthly things’ tend to fall towards earth; solar things tend to fall towards the sun; Martian things tend to fall towards mars and so on and for forth. What he meant was ‘I haven’t a bloody clue’, thus demonstrating a good scientific theory doesn’t always need to make any sense.

But by far the biggest challenge to heliocentrism was the parallax, the way nearer objects will seem to move relative to more distant objects. Copernicus’s contemporaries argued that if the earth moved around the sun, then there must be some evidence of a parallax effect; in fact the same objection had been raised against Aristarchus centuries earlier. A moving earth would mean that the earth would be on opposite sides of the sun six months apart. And if the earth was on opposite sides of the sun six months apart there should be some visible change in the relative position of the stars during the year. The only way Copernicus could preserve his model was by contending that the stars were so far out that a stellar parallax would not be measurable. Thus the heliocentric universe would have to be massively larger than otherwise necessary; a very ad hoc explanation.

As a result you can count on the fingers of one hand the number of Copernicus’s contemporary astronomers who joined him in espousing heliocentrism. It is not true, as Arthur Koestler maintained in ‘The Sleepwalkers’ that De revolutionibus was ‘the book no-one read’. In fact, as Owen Gingerich has shown, despite being a murderously technical and geometrical treatise it was owned by virtually all of the important figures in the history of astronomy. It was not rejected on the grounds of close-mindedness, biblical literalism or conservatism. It was rejected based on deeply held scientific principles. It was used for calculations but it was thought to be physically impossible.

The Copernican System was complex and unwieldy, requiring a total of 48 epicycles, compared to 40 in the Ptolemaic geocentric system. It did however have a number of minor advantages which would become compelling as time went on. It had a slight edge over the Ptolemaic system in predicting the positions of the planets and it eliminated the clumsy equant. The biggest asset it had was its conceptual simplicity, eliminating many of the contrivances of the geocentric system and the differences in the motion between inferior and superior planets. In time it would become powerful and truly revolutionary.

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