Weird Earth. Donald R. Prothero
among themselves, as Plato had done with the philosophical debates of Socrates. Galileo could put his own ideas in the words of one character (Salviati, the scientist in the dialogue) but could plausibly claim that he himself wasn’t advocating heliocentrism; the heretical ideas were spoken by only one character in his “play.”
Salviati presents the Copernican arguments for heliocentrism in a debate with the other main character, Simplicio, who represents the traditional geocentric views of Ptolemy, Aristotle, and the Church. Although the name was drawn from Simplicius of Cilicia, a sixth-century commenter on Aristotle, it was a deliberate double entendre, since Simplicio also meant “simpleton” or “fool” in Italian. The third character in the play is Sagredo, an intelligent but uncommitted layman and merchant who acts as the target audience and jury for the arguments of the other two. He eventually agrees with Salviati’s heliocentric solar system—as would any reader, since Salviati demolishes Simplicio’s arguments.
Figure 3.2. Famous 1888 engraving by Flammarion showing the old geocentric view of the universe. The image depicts a man crawling under the edge of the sky, depicted as if it were a solid hemisphere, to look at the mysterious Empyrean beyond. The caption translates to “A medieval missionary tells that he has found the point where heaven and Earth meet.” (Courtesy Wikimedia Commons.)
Galileo thought that he had cleverly avoided being accused of directly advocating heliocentrism as a true description of the world, but the Inquisition was not amused. The pope did not like the public ridicule that was clear in the character of Simplicio. Galileo was called to Rome to face several days of trial, where they showed him instruments of torture and made him recant his views on pain of torture and death. As he bowed before them and confessed his rejection of the heretical heliocentric views, Galileo supposedly muttered under his breath, “Eppur si muove” (“And yet it moves!”). Rather than torture the old man, the pope sentenced him to house arrest, where he spent the last ten years of his life unable to leave his domain. There, Galileo wrote his last great work, Discourse and Mathematical Demonstrations Relating to Two New Sciences, where he laid down the foundations of modern physics, especially regarding the motions of objects (kinematics).
By 1638, Galileo was completely blind (partially due to staring directly at the sun with his crude telescope to see the sunspots) and was suffering from a painful hernia and insomnia, and his devoted daughter had to take care of him. He died on January 8, 1642, at the age of seventy-seven. His Dialogues, along with Copernicus’s work, was banned by the Church, so they could not be printed or read except in places where the Church’s power was limited. Galileo’s books and his ideas were still under official Church ban until 1835, even as the rest of the world had moved on to modern astronomy thanks to the work of Isaac Newton in the early 1700s. Finally, on Halloween 1992, Pope John Paul II officially acknowledged the errors of the Catholic tribunals and announced that a statue of Galileo would be placed in the Vatican. In December 2008, Pope Benedict XVI praised Galileo’s work on the four hundredth anniversary of Galileo’s earliest telescopic observations. However, the plan to put his statue in the Vatican has since been shelved.
The idea of a heliocentric solar system is most famously attributed to the Polish scholar Mikolaja Kopenika (in Polish; we know him by the 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