In the comments column to my last post Michael Fugate asked the following questions:

In 1610, what was the evidence that supported geocentrism over heliocentrism?

Tradition?

Had the church proved that geocentricity was true?

Tim O’Neill and Anger Bear have already given good partial answers in the comments but I have decided to give a longer fuller answer here.

The answer to all of Michael’s questions can be summed up in four terms, tradition, common sense, physics and parallax, which I shall expand upon in that order.

Tradition plays a big role in the history of science and in particular in the transition from one major theory to a new one. An old established theory is not given up just because there is a new brash kid on the block. The situation is best summed up in a famous quote that is often falsely attributed to Thomas Kuhn but is in fact from Max Planck, who was, himself, famous for introducing a major new theory in physics:

A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.

This traditional resistance to change, a form of inherent conservatism, actually serves a useful purpose in the evolution of science. In championship boxing in a title match the challenger, if he wishes to win the title, is expected by the point judges to do more than the champion. He must be more aggressive, he must throw more punches and in general he must be significantly more active than the reigning champion. The theory is that the champion has already proved his worth, after all he is the champion, and the challenger must show that he is truly worthy to replace him. Being as good is not enough he must be better. The same is true of scientific theories an established theory has already shown that it can explain the phenomena covered by the theory and has over a given period of time proved its worth. A new theory must show that it can explain the phenomena better than the old theory and also stand up to a thorough critical examination, these things take their time and a change of theories does not take place overnight. The transition from geocentrism to heliocentrism took about one hundred and twenty years, which is about par for the course for such a large theory. When I was growing up in the 1950s and 60s relativity was well on the way to being established but it was still considered OK to cast doubts on its validity, quantum mechanics was definitely left field and very suspect. I can remember as the first tests of time dilatation were carried out using pairs of atomic clocks and a jet bomber. Nowadays both theories are completely accepted almost without question in both cases more than one hundred years after quantum theory and special relativity first saw the light of day.

In 1610 geocentrism had been the established theory of the universe in Europe since the Pythagoreans in the 6th century BCE. Eudoxus, Plato, Hipparcus, Aristotle and Ptolemaeus had all accepted that the earth was stationary and at the centre of the other celestial bodies that circled it. They all had different explanations of the mechanism of the system but the fundamentals were the same. In the period between the 8th and 15th centuries many Islamic astronomers and philosophers worked on those models but none of them questioned the basic facts. Also as a system for predicting the movements of the planets and the stars the Ptolemaic model had proved remarkably efficient for fourteen hundred years. If heliocentrism was going to usurp the throne it would have to be significantly superior. In fact it failed the first test as tables based on De revolutionibus proved to be no better than those based on the Syntaxis Mathematiké. In fact it was only when Kepler produced table based on the data of Tycho and his own planetary model, which were significantly superior to everything that had gone before that heliocentricity began to find widespread acceptance.

Tradition, in this case, was of course strongly supported by common sense. If you go out on a dark night somewhere where this is no light pollution and look up at the sky for a long period of time you should be able to note a series of things. Firstly the earth on which you are standing is rock solid and does not move. Secondly the stars all appear to be attached to a great sphere, which is slowly rotating about you at the centre. Thirdly the planets, a small number of especially bright and eye-catching stars, are also rotating around you at the centre of their apparent orbits. Congratulations you have just collected empirical evidence for geocentricity. Common sense, in this case literally that which we perceive with our senses tells us that the universe is geocentric, which brings us to physics.

Using the term physics here is not without problems. I am using it in the modern meaning of the term but at the beginning of the 17th century physics as we know it didn’t exist, which as we shall see is the whole problem, and the word physics meant something completely different in Aristotelian philosophy. Imagine you are sitting on a motorbike on a smooth straight road, you accelerate to 60 kph and then being a skilled and confident biker you take your hands from the handlebars and sit up straight. Now you take a newspaper out of your pocket and read it. This would of course be impossible, the headwind would blow the paper out of your hands and if you did manage to hold it tightly enough to spread it the wind would tear the paper in half. You were only travelling at 60 kph. If the world was spinning on its axis as dictated by heliocentrism someone standing on the equator would be whirling round not at 60 kph but in excess of 1600 kph! Even worse the whole world would be hurtling through space at over 100 000 kph! Now come on who are you kidding? Pull the other one it’s got bells on! Now of course people at the beginning of the 17th century did not have motorbikes, or newspapers for that matter, but they did have horses and carriages and knew all about headwinds. If the earth is moving at such frightening speeds why isn’t everything on it blown away? Copernicus already knew the correct answer to this problem. The earth and everything on it is contained in an envelope, we call it the atmosphere, which travels with the earth so there are no headwinds. However the physics necessary to explain this model didn’t exist in 1610. There was no concept of force or of mass, no correct definition of inertia let alone gravity how was this envelope supposed to work? What held it in place? Why wasn’t it blown away? The physics necessary to answer these questions was developed over the 17th century. Galileo in his Discorsi, who gave the laws of fall, was one of those along with Kepler, who gave the first primitive definition of force and the concept of gravity, Stevin, who first described the vector parallelogram of force before there were vectors, Beeckman, who gave the correct definition of inertia, and others who laid the foundation of physics on which Newton would build the theory of universal gravity and thus give scientific substance to Copernicus’ envelope. There are other physical problem thrown up by heliocentrism that had to be and were solved in the course of time but I think the one example is enough to show that heliocentricity was not ready to become champion in 1610. Interestingly the Jesuit astronomer Riccioli realised that if the earth rotated on its axis then, that which we call, the Coriolis effect must exist. He set up experiments to detect and because he failed to do so he concluded that the earth does not move.

Astronomically the big bummer was stellar parallax. If the earth rotated around the sun then it must be possible to detect stellar parallax when viewing the stars from opposite extremes of the earths orbit. All attempts to do so failed miserably up to the 1820s a very black mark against heliocentricity. In fact heliocentricity was accepted long before stellar parallax was finally detected. However at the beginning of the 17th century this failure weighed heavy in the arguments against heliocentricity. The alternative and true explanation that the nearest stars were so far away that the parallax was too small to be detected with the available instruments meant distances so great as to be literally inconceivable at that time. Only over time as the accepted dimensions of the universe grew larger and larger did this alternative become plausible.

All in all to believe in heliocentricity at the beginning of the 17th century was literally an act of blind faith and those that opposed it did so on solid scientific grounds and not purely out of some sense of religious bigotry as is often claimed by those who don’t know their history of science.