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Galileo and the Leaning Tower of Pisa ✸ 13 2 Galileo’s Pythagorean Heresy ven without evidence, most people believe in Earth’s motion. When asked why Earth moves, some college students reply: “because of the seasons” or “because astronauts see it from spaceships.” But seen from Earth, the spaceships seem to move instead, so who’s right? And for thousands of years, it seemed that the seasons change just because the sun rises at different places on the horizon, taking different paths; so why not say that seasons are caused by the sun’s motions? Centuries ago, teachers taught that the sun circles Earth, and it seemed obvious; now they teach the contrary, and it seems obvious. It’s easy to accept basic scientific knowledge without knowing why scientists believe it, but that would make science seem like just another doctrine. Many myths are associated with the Copernican revolution. I’ll now recount the dramatic struggles by which people changed their minds about Earth’s motion, while also pointing out several myths. But mainly, I will trace the many connections to legends about Pythagoras and his followers. For centuries, writers have often associated the idea of Earth’s motion with Pythagoras of Samos, a philosopher who lived in southern Italy in the sixth century BCE.But is there truth to such claims? Strangely , I find not a single historical account that systematically connects the diverse legends about Pythagoras and his followers to the Copernican revolution. Making that connection reveals a surprising dimension of Galileo’s famous heresy. According to Aristotle, the philosopher with the aquiline nose, the so-called Pythagoreans believed that Earth moves in circles around the center of space:“they say that fire is at the center and that Earth is one of the stars, and that moving in a circle about the center it produces E 13 night and day.”1 But their arguments did not convince Aristotle, who had many good arguments to the contrary. For example, he proposed, if Earth moves across the heavens, then we should observe certain changes in the background of stars. Specifically, we should see apparent changes in the gaps between the stars. Consider a few objects in a row, as seen by someone running along. There’s a line of sight to each object. The objects closest to the person will seem to be farther apart from one another, the two objects farthest away will seem to be closest together. But once the person has reached those farthest objects, the effect will be reversed.2 Likewise, if Earth moves across the heavens, we would expect that the relative distances between the stars should change. Ancient astronomers could expect to see such effects, and more, if Earth were moving. But no such effects were observed. So Aristotle fairly argued that Earth does not move at all. Still, decades after Aristotle’s death (in 322 BCE), the astronomer Aristarchus of Samos advocated that Earth travels in circles around a center, the sun. No treatise by Aristarchus (who died around 230 BCE) on this matter seems to have survived, but there are brief, indirect accounts . In particular,Archimedes mentioned the theories of Aristarchus in this address: Figure 2.1. Lines of sight to a group of objects. As the man moves, the gaps between the objects seem to change 14 ✸ Galileo’s Pythagorean Heresy You, King Gelon, know that‘universe’ is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus brought out a book consisting of certain hypotheses , wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the‘universe’ just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.3 Archimedes rejected Aristarchus’s theory, as did most mathematicians and astronomers. One problem was...


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