Latinized version of his name, Nicolaus Copernicus), although a version of the idea was first proposed by the Greek scholar Aristarchos of Samos about 280 BCE. Copernicus was a true genius; fluent in Latin, German, and Polish, and also speaking some Greek, Italian, and Hebrew, he often worked as a translator. He is more famous as an astronomer and mathematician, although he also served as a diplomat and governor. As an economist, he formulated the quantity theory of money, an idea that later came to be known as Gresham’s law.
But Copernicus is most famous for making the astronomical observations that led him to suggest heliocentrism and give strong evidence for it. In the years 1512–1515, he made a long series of intensive observations of stars and planets. Among the puzzles that he dealt with was a curious phenomenon known to the ancient Greek astronomers as retrograde motion. If you studied the position of certain planets, like Mars and Jupiter, against the background of the “fixed stars” night after night, you would observe something odd. Each night, the planets seem to have moved farther in the sky than the previous night, as if they were circling the earth. But once in a while, the planet appeared to pause, then back up a short distance, before resuming its former forward motion. This backward, or “retrograde,” motion (fig. 3.3A) made no sense if planets simply orbited the earth in a simple circle (or, later, an ellipse).
Many solutions to this puzzle were proposed, but the most famous was by the Hellenistic Greek astronomer Claudius Ptolemaeus (known as Ptolemy today) who lived in Alexandria around 100–170 CE, during the days of the early Roman Empire. He viewed the universe as a set of nested spheres, all spinning around the earth at the center (fig. 3.2). Each of the planets was on a sphere spinning around the earth, and the “dome of the sky” covered with the “fixed stars” was the outermost shell of the spheres. His Almagest summarized nearly all the known observations of the stars and planets up to that time, so it was the foundation of all later astronomy.
To explain the odd backward motion of certain planets, he postulated that they didn’t move in a single circular path; instead, they were moving around a small circle (epicycle) whose center was the larger circle of their motion around the earth (fig. 3.3B). Sometimes during their motion around the earth, they would be on the reverse-moving part of the epicycle, so they would appear to move backward from the perspective of the earth. This idea was soon the most popular among all the astronomers. By the time the Church dominated all Western thought, they made Ptolemy’s system the officially approved model of the universe, just as Aristotle’s ideas about nature were also considered officially sanctioned by the Church. For over 1,400 years, no one dared challenge the Ptolemaic system.
Copernicus was dissatisfied with Ptolemy’s explanation of retrograde motion but not because he was a rebel who wanted to challenge the Church or the dogma of his day. Instead, he disliked Ptolemy’s system of epicycles because it seemed too complicated and inelegant. He sought a simpler explanation that made sense without all the tinkering and fudging that astronomers had to do to make the epicycles work. Eventually, Copernicus realized that if the sun, rather than earth, was the center of the system, it all made sense. If Mars and Jupiter were on orbits around the sun but outside earth’s orbit, then they would be moving in much bigger circles and much more slowly than us.
Figure 3.3. Retrograde motion. A. The apparent motion of asteroid 514107 2015 BZ509 against the background of fixed stars as it is viewed night after night. B. Ptolemy’s explanation of retrograde motion, where planets move in epicycles centered around a point in orbit around the earth. C. Copernicus’s explanation, where apparent retrograde motion is caused when the faster-moving earth (numbers 1–5) on a short inner track catches up and passes a slower-moving outer planet like Mars or Jupiter (dots on outer track). The projection on the right shows how the outer planet would be seen from earth. It appears to backtrack as the earth passes it on the inside track. (Courtesy Wikimedia Commons.)
Let’s imagine that Mars or Jupiter is ahead of us in its orbit (fig. 3.3C) and the earth comes up fast behind them, around its much shorter inside orbit, until it passes Mars or Jupiter. From the earthbound perspective, Mars will appear to move forward then appear to back up as we overtake it on the inside bend. After we pass it completely, it will appear to move forward again. It’s analogous to two race cars going around a big curve. The car on the inside of the bend has less distance to travel, so it will often pull ahead of a car leading it on the outside bend. From the inside driver’s perspective, the outside car appears to slow down and fall back as it is passed, even though from the spectators’ view, all the cars are moving forward. We travelers on the race car that is earth are on a faster-moving vehicle on the shorter inside bend from outer planets like Mars and Jupiter, so each time we come up from behind and pass them, they briefly appear to be falling back.
This simple but elegant solution first came to Copernicus shortly after his observations in 1515, but reluctant to publish, he spent a lot of time writing it up and tinkering with it. Some of his students wrote and published short summaries of his ideas, so they were known to many scholars and some Church officials. Copernicus himself wasn’t in any hurry until late in his life, when he finally wrote it all down as De Revolutionibus Orbium Coelestium (“On the Revolution of Heavenly Spheres”). Copernicus was justifiably afraid of criticism from the Ptolemaic astronomers of the time and especially from the Church—so much so that he dedicated the work to Pope Paul III in hopes of placating religious authorities. The book finally went to press in 1543, just as Copernicus was dying of a stroke and paralysis at age seventy. Legend has it that he was shown the final printed pages of his book just before he died, knowing that his work would be published.
Once he died, Copernicus was safe from the firestorm of anger, criticism, and censure that his work provoked and from the torture of the Inquisition. After his death, his work was mostly ignored as purely theoretical for decades, and not until people like Giordano Bruno (burned at the stake for his heretical views) and Galileo revived it did the Church consider his work a threat and ban Copernicus’s book. As we just discussed, it was not until the 1990s that the Church finally made official peace with Copernicus and Galileo, even though their work had become the foundation of modern astronomy with the work of Newton in the early 1700s.
“Galileo Was Wrong: The Church Was Right”
Most readers of this book might be thinking, “OK, so much for the history of how the heliocentric solar system was discovered. After all, scientists proved it in the 1700s, and even the Catholic Church finally recanted, only 350 years late.” That’s what I thought too, until I was startled to find mention of a seminar held at Notre Dame University in South Bend, Indiana, on November 6, 2010, titled “Galileo Was Wrong: The Church Was Right.” At first I thought it might be some kind of clever satire, but once you clicked on the web page (since taken down), it was clear that they were dead serious! Who were these people, and how is it that they have a significant following in the twenty-first century?
Robert Steinback of the Southern Poverty Law Center, which monitors racists and antisemites, attended the meeting and described it as follows:
“Seminar” might be generous phrasing: The presentation was a mind-numbing, 15-hour-long sermon-cum-pep rally for radical traditionalist Catholic apologists desperate to debunk any science that suggests the Bible shouldn’t be interpreted literally. Numerous biblical passages describe the earth as at rest, with the sun in transit around it. About 90 mostly Catholic devotees, curious
