Nicholas Copernicus’ book On the revolutions excited no particular controversy when it was published. Very few sixteenth-century readers accepted the physical reality of the Copernican system (for the reasons I discussed in Responses to Copernicus). However, astronomers all over Europe read the book with considerable interest and enthusiasm. Many extant copies of On the revolutions, including the copy held by the University of Oklahoma History of Science Collections, have quite extensive marginalia, indicating that they were read carefully and closely by readers interested in coming to grips with the mathematical models Copernicus proposed. (For more on early readers of Copernicus, listen to the NPR podcast on this topic by Owen Gingerich.) Many of these astronomers were perfectly happy to use Copernicus’ mathematical models for calculating planetary positions. They were useful for such practical activities as astrological predictions and the reform of the calendar. Not until the late sixteenth century did any astronomer find any new observational evidence that cast any doubt on the physical reality of the Aristotelian/Ptolemaic view of the cosmos. The first astronomer to do this was Tycho Brahe. The second was Galileo Galilei, whose work I will describe in more detail in this page. Before I get into Galileo’s work, let me say a little bit more about the similarities and differences between the Copernican and Ptolemaic models.
Probably the most famous image from Copernicus’ On the revolutions is his diagram of the heliocentric cosmos.
To modern readers and students, this often looks deceptively simple. You’ve learned about the complex mathematical models that Ptolemy used to account for the varying speeds with which celestial bodies moved around the earth (eccentrics, epicycles and equants) and to account for the retrograde motion of the planets (epicycles).
To twenty-first century eyes, these models often seem unnecessarily complicated and the obsession with perfect circles little short of bizarre. Unfortunately, the impression that these models are ridiculously complicated is furthered by a good bit of misinformation and misunderstanding of Ptolemy’s models. For example, watch this video on Ptolemy and Homer Simpson.
The video begins with the statement that, “it is possible to reconstruct any planet’s orbit with Ptolemy’s system of epicycles.” This suggests that Ptolemy himself, or his successors, used epicycles on epicycles to construct mathematical models that would be accurate. But while they may be accurate, it’s hard to believe they could be physically real. The idea that Ptolemy’s models are bloated with epicycles has even seeped into some textbooks by historians of science (who ought to know better!). Take for example, the description of Ptolemy’s Almagest in Andrew Ede and Lesley Cormack’s A History of Science in Society: From Philosophy to Utility:
Although much of the Almagest was complicated, part of its power was that it was not mathematically complex. All the elements of Ptolemy’s models were based on the geometry of the circle, which was well understood. While there could be many epicycles employed to establish the orbit of a planet, they were all constructed the same way. The Almagest provided a complete account of celestial motion of all the objects that could be seen with the naked eye. (pp. 35-36)
There is much in this passage that is accurate: Ptolemy’s models are based on the geometry of circles and they are astoundingly accurate (at least for naked eye observations). However, there are two glaring errors, both in the same sentence: “While there could be many epicycles employed to establish the orbit of a planet, they were all constructed the same way.” First, Ptolemy’s models do not describe the ORBITS of planets. The orbit (from Latin orbita, which means “a track or rut made in the ground by a wheel”) is the path of a planet through space – or the line traced by the planet as it moves through space. Remember that Ptolemy is NOT describing ORBITS; he is describing ORBS. Each planet is embedded in an orb, and the orb rotates around the earth. The second error is that Ptolemy NEVER used more than one epicycle to account for a celestial body’s motion. All of his models contain one and only one epicycle.
Let’s return to Copernicus’ model. It might seem that he would not need to utilize the mathematical devices of the eccentric, epicycle and equant in his system, because the varying speeds with which the celestial bodies APPEAR to move and the APPARENT retrograde motion of the planets can be accounted for by the motion of the earth around the sun. However, this is incorrect. Like Ptolemy, Copernicus had to employ eccentrics and epicycles in his models in order to get them to fit observational data. Unlike Ptolemy, Copernicus did NOT use the equant because he believed this device did not conform to the requirement of uniform circular motion. And Copernicus is even more devoted to motion in the heavens being composed of perfect circular motions than Ptolemy was.
In the early seventeenth century, the Italian mathematician and astronomer Galileo Galilei (1564 – 1642) made some remarkable observations of the heavens that cast serious doubt on the validity of the Aristotelian/Ptolemaic view of the cosmos. Galileo was one of the very first people to observe the heavens through a telescope. In 1609, Galileo heard reports of an optical device that could magnify distant objects, a device most likely invented by the Dutch spectacle-maker Hans Lipperhey. Based on these reports, and his knowledge of optics, Galileo constructed his own telescope, and in 1609 began making observations of the heavens. (He was not the very first person to do this. The English astronomer Thomas Harriot made observations of the moon through a telescope about four months earlier than Galileo.) Within the space of a few months, Galileo had made three startling discoveries with his telescope. He announced these discoveries in a book called The Starry Messenger, published in 1610. Galileo’s discoveries did NOT constitute proof that Copernicus was right and the sun really was in the center of the universe (and Galileo did not try to claim this in 1610), but they did raise serious challenges to the accepted view of the cosmos.
I include below several pictures from the very richly illustrated Starry Messenger. All of these images are from the copy of the Starry Messenger in the OU History of Science Collections. You can see more of the book on the OU History of Science Collections Flickr site. For an explanation of the contents of the Starry Messenger and their implications, read The Starry Messenger: What it Said and What that Really Meant! by Thony Christie. Please note, this is REQUIRED READING.
For an account of Galileo’s clash with the Catholic Church, read Thony Christie’s Galileo, the Church and Heliocentricity: A Rough Guide. This is REQUIRED. If you want more information, you can also read Christie’s Galileo, Foscarini, The Catholic Church, and heliocentricity in 1615 Part 1 and Part 2. (These are optional.)
Postscript: This is just creepy and weird, but I can’t resist including it. In Florence, Italy there is a fabulous history of science museum with extensive holdings connected to Galileo. The Museo Galileo is well worth a visit if you are ever in Florence, and even if you’re not, they have a sensational website. The strangest thing they have is Galileo’s middle finger, preserved in a lovely glass egg with gilt trimmings on an alabaster stand. See here for the inscription on the base. I have no idea why they have this. Perhaps it’s Galileo’s last gesture to the pope?
References and useful links:
Andrew Ede and Lesley B. Cormack, A History of Science in Society: From Philosophy to Utility, 2nd rev. ed. (University of Toronto Press, 2012).
Before Newton 