Myths about the Copernican Revolution

Painting of Copernicus The frontpiece of G.B. Riccioli's Almagestum Novum, 1651.
Painting of Copernicus by Jan Matejko from the end of the 19th century The front piece of G.B. Riccioli's Almagestum Novum, 1651. Notice that he does not use Tycho Brahe's system but his own variation of it where Jupiter and Saturn are centered on the Earth.

There are a lot of myths about the Copernican Revolution. Many people whose main interest is philosophy of science, but with little knowledge of astronomy or history of science, have written extensively about it. Unfortunately, much of what they say is incorrect.

Myth: The Ptolemaic system was in a crisis. Many writers claim that the Ptolemaic system required epicycle upon epicycle to correct errors. This is not true. The reason why some Arab astronomers introduced epicycles on epicycles, was to avoid using an equant point. Moreover, introducing epicycles would only solve periodic errors, not secular errors. The best reference for this is the article by Owen Gingerich, “Crisis” versus Aesthetic in the Copernican Revolution, from his book The Eye of Heaven: Ptolemy, Copernicus, Kepler.

Myth: The Copernican system was simpler than the Ptolemaic system. Some things were clearly simpler, but he actually used more epicycles! The Prutenic Tables were not better than the Alfonsine tables.

Myth: Epicycles were absurd. Epicycles were a brilliant idea! To an observer on Earth, the motion of the planets will look like “epiellipses” on elliptic deferents. Epicycles give good results for Venus, Jupiter and Saturn. Mercury and Mars have larger eccentricities, so the model was not so good for them.

Myth: Galileo's discovery of the entire cycle of phases of Venus showed that the Universe is not geocentric. What do you mean by geocentric? Many people think of a geocentric system as just meaning that the heavenly bodies move around the Earth as in this applet. Of course, this makes no astronomical sense, since in that case Mercury and Venus can both be in opposition to the Sun! One essential assumptions in Ptolemy's Almagest is that the center of the epicycles of Mercury and Venus are collinear with the Earth and the Sun. However, even that isn't enough to give a contradiction, because we could make the centers coincide with the Sum, essentially giving a geostatic, but heliocentric system, similar to Tycho Brahe's system. We could also make the radius of the epicycles big enough to sometimes take the planet outside the orbit of the Sun. (Not very classical, but logically possibly.) It's only if we consider geostatic to mean the Ptolemaic system as laid down, not only in the Almagest, but in his book Planetary Hypotheses in which he specifies the dimensions of the system, that the phases of Venus becomes a clear contradiction. However, Galileo's discovery was not the first problem for the Ptolemaic system. Ibn al-Shantir had criticized and corrected his lunatic lunar theory, which would have made the Moon twice as big at quadrature as at syzygy.

Myth: Galileo's discovery of the entire cycle of phases of Venus was proof of the truth of the Copernican system. The phases of Venus went against the Ptolemaic system, but not against Tycho Brahe's system. In particular, I do not know of any astronomer who converted to the Copernican system because of Galileo's discovery.

Myth: The battle was between the Copernican and the Ptolemaic systems. Tycho Brahe introduced his geostatic, heliocentric system in 1588, and it quickly won the support of the Catholic Church. It is very interesting that Galileo does not discuss Tycho's system in his 1632 book. Probably because he realized that it would be very hard for him to argue against it!

Myth: Once Copernicus has proposed his system, it was stupid not to believe in it. It took a long time before Copernicus's system was accepted.

Myth: It is easy to see that the Earth moves around the Sun. I'm serious! There are people who claim this! Newton's Principia from 1687 gave people some reason to believe in a heliocentric system, but it was only with the discovery of aberration in 1725 (published in 1728) and Bessel's discovery of stellar parallax in 1838 that there was actual proof.

Myth: The only difference between Copernicus's system and Ptolemy's is a change of reference frame. Copernicus was the first person to determine the relative distances between the planets. For more details, read Giorgio Goldoni, Copernicus Decoded, The Mathematical Intelligencer, 27 (2005) 12-30 or The Copernican Revolution and the Size of the Universe by my student LU Tian Xin.

Myth: Copernicus's main contribution was suggesting a heliocentric system. With hindsight this is of course true, but his contemporaries were most impressed by the way he had removed the equant point and his use of eccentrics with compound, small epicycles and Tusi couple.

Myth: The Catholic Church was against science. The Church was clearly wrong in the way it handled Galileo, but he was not easy to deal with! Many Jesuit astronomers did excellent work. In particular they used meridian lines to verify Kepler's bisection of the eccentricity.

I hope to expand on this page. In the meantime, here are some excellent books.

You may also enjoy the page for my course Heavenly Mathematics & Cultural Astronomy and A guide to “The Sun in the Church” by J.L. Heilbron.


Helmer Aslaksen
Department of Mathematics
National University of Singapore
helmer.aslaksen@gmail.com

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