Today is the birthday of Nicolaus Copernicus, in Royal Prussia in 1473. A polyglot and polymath, he obtained a doctorate in canon law and was also a mathematician, astronomer, physician, classics scholar, translator, governor, diplomat, and economist. In 1517 he derived a quantity theory of money—a key concept in economics—and in 1519 he formulated an economic principle that later came to be called Gresham's law.
But he is, of course, most famous for the heliocentric model of the universe. He did NOT invent the idea, however. The idea has a long back story, beginning with Aristarchus in ancient Greece.
Through some quite clever observations, mostly of eclipses and the phases of the Moon, Aristarchus created a series of ratios of the sizes of Earth, Moon and Sun. While he needed, and didn't have, an actual number to put in to work out the exact sizes (that wasn't to come until Eratosthenes paid a student to walk from Alexandria to Aswan and count how many steps it took), what he did know was that the Sun was vastly larger than the Earth. It made no sense to him that something that large should be subject to something that small, and so he was the first heliocentrist.
Others immediately realized what the flaw was here. If the Earth moves, then your viewpoint on the stars is changing as it does so. You should see them in somewhat different directions at different times of year. This is called parallax. Each would trace out a tiny replica of the Earth's orbit, in the sky. That didn't appear to happen. Aristotle, among others, realized that there were only two plausible explanations. Either the universe is vastly larger than anything anyone has ever had experience with so the movement is so tiny that you can't see it, or the Earth doesn't move. The latter explanation seemed simpler.
After Aristotle was imported wholesale into Catholicism by Thomas Aquinas, that became Church dogma as well, supported by some vague references in the scriptures. There was also a diktat that the only motion suitable for the heavens was circular motion, since the circle is more perfect than any other geometric form. It has the largest number of symmetries, and symmetry appeals to the primate brain.
So in retrospect, the Ptolemaic system is an attempt to make circles centered on the Earth behave like ellipses with the Sun at one focus. In technical terms, it is a Fourier expansion of an ellipse. And it went like this.
The largest contribution to he movement of the Sun and planets is almost on a large circle, a cycle, sort of centered (more on that in a moment) on the Earth. Periodically, the planets appear to move backwards in the sky. This is called retrograde motion. Then they move forward again. To account for this, the planet goes around a smaller circle, an epicycle, whose center is moving around the cycle. So periodically the epicycle carries the planet backwards as the epicycle itself is moving forward.
But not all times of retrograde motion are the same. They vary a bit. To account for this, the large circle, rather than being centered on the Earth, is actually centered on a point slightly away from the Earth, called the equant. The Earth is still stationary at the center of the universe, but the planets do not orbit around the center of the universe, rather at a point slightly off. So sometimes the retrograde motion is closer to us than at other times, accounting for the variations.
There is no explanation for why any of this happens. In fact, ancient philosophers denied that there ever could be one. The purpose of astronomy was not to explain things but to "save the appearances." It isn't even clear whether many of them believed it was real, rather than just a conceptual machine that more or less did the same thing as reality.
Copernicus is said to have been motivated by the fact that the Ptolemaic system got increasingly out of kilter over the centuries. That is wrong. It worked as well as it ever did, which is to say tolerably well as long as you tuned it up periodically. Rather, according to science historian David Park in The How and the Why (highly recommended), he was motivated by aesthetic hatred of the equant. It made the whole sky lurch backwards and forwards, violating his sense that things should move smoothly.
Copernicus' heliocentric system does a few things. It moves the Sun to the center, doing away with the need for an equant. The planets' orbits themselves will take care of them being closer or further away. It embraces the idea of a truly huge universe to deal with the parallax problem. It imagines that the Sun, while at the center of the planetary system, may indeed not be at the center of the universe. But it retains one critical feature of the Ptolemaic system: only circular motion is allowed. Since the planets actually move on ellipses, and when they are far from the Sun move a bit slower than when they are near, Copernicus had to retain the system of epicycles. They were very much smaller since they didn't have to account for retrograde motion, but they were there.
Copernicus wrote a summary of these ideas in the Commentariolus (Little Commentary) and circulated it to his friends. A year later, he completed a detailed descrition, Dē revolutionibus orbium coelestium (The Revolutions of the Celestial Orbs). His friends urged him to publish, but he feared public ridicule, not punishment from the Church.
Far from it. In 1533, Johann Albrecht Widmannstetter delivered a series of lectures in Rome outlining Copernicus's theory. Pope Clement VII and several Catholic cardinals heard the lectures and were interested in the theory. On 1 November 1536, Cardinal Nikolaus von Schönberg, Archbishop of Capua, wrote to Copernicus from Rome:
"Some years ago word reached me concerning your proficiency, of which everybody constantly spoke. At that time I began to have a very high regard for you... For I had learned that you had not merely mastered the discoveries of the ancient astronomers uncommonly well but had also formulated a new cosmology. In it you maintain that the earth moves; that the sun occupies the lowest, and thus the central, place in the universe... Therefore with the utmost earnestness I entreat you, most learned sir, unless I inconvenience you, to communicate this discovery of yours to scholars, and at the earliest possible moment to send me your writings on the sphere of the universe together with the tables and whatever else you have that is relevant to this subject .."
There are a couple of things that probably turned the church against the idea. First, in 1539 Georg Joachim Rheticus, a Wittenberg mathematician, arrived at Copernicus' home in Frombork. Philipp Melanchthon, a close theological ally of Martin Luther, had arranged for Rheticus to visit several astronomers and study with them. Rheticus became Copernicus's pupil, staying with him for two years and writing a book, Narratio prima (First Account), outlining the essence of Copernicus's theory. Under strong pressure from Rheticus, and having seen the favorable first general reception of his work, Copernicus finally agreed to give De revolutionibus to his close friend, Tiedemann Giese, bishop of Chełmno (Kulm), to be delivered to Rheticus for printing by the German printer Johannes Petreius at Nuremberg (Nürnberg), Germany. While Rheticus initially supervised the printing, he had to leave Nuremberg before it was completed, and he handed over the task of supervising the rest of the printing to a Lutheran theologian, Andreas Osiander. So teh Copernican system slowly made its way into the protestant side of the Reformation/Counter Reformation disputes.
It didn't really hit the fan, however, until the belligerent and arrogant Galileo became a convert. Up until that point, the idea was still the ancient Greek one -- you're not describing reality, you're just saving the appearances of reality. Galileo was the first to assert that heliocentrism was really real, and pointed to his telescopic observations of the phases of Venus and the moons of Jupiter. That brought it into direct conflict with holy scripture and holy Aristotle, De Revolutionibus was placed on the Index of Prohibited Books. Later, the Index itself was placed on the Index as it seems some enterprising Florentine souls were using it as a guide to interesting reading.
So there you go. Everything they told you about the history of heliocentrism in middle school is wrong, a Just So story.
Oh, and retrograde motion? When you pass a car on the freeway, it moves backward relative to you. Same thing. When the Earth passes another planet, it appears to move backward.
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