Tycho and Kepler Read Online Free Page B

the carousel turns, the horses not only make their way all the way around the large circle but also move in smaller circles, chasing their tails.
    By the same token, in a stroke of insight, ancient astronomers realized that if the planets moved continuallyin smaller circles centered on the rim of a larger circle centered on Earth, the result would be the regularly occurring retrograde motion they were observing.
    The technical term in Ptolemaic astronomy for the small circle in which a planet moved was
epicycle
. The larger circle on which the epicycles turned (in figure 1.2 , the inner circle—the radius at which the minicarousels are bolted tothe floor) was the
deferent
. By adjusting the size, direction, and speed of the epicycles, astronomers could explain many irregularities they observed in the way the planets, Sun, and Moon move. A planet traveling in its epicycle would sometimes be closer to Earth and sometimes farther away, which explained apparent variations in its brightness. In Ptolemaic astronomy a planet’s sphere was justlarge enough for the planet to cartwheel along on its epicycles.
    Tycho, Kepler, and their peers at university also learned the use of the
eccentric
. With an eccentric, the planet (perhaps simultaneously traveling in an epicycle) orbited Earth, but the orbit wasn’t centered precisely on Earth. Its center was a point a small distance away from Earth.

    Figure 1.3: Devices of Ptolemaic astronomy: eccentric orbit, deferent, epicycle, and equant.
    Epicycles, deferents, and eccentrics were devices Ptolemy refined from earlier astronomy, but another, the
equant
, was probably his own invention. Many astronomers were uncomfortable with it, for it was not only complicated to use but also seemed to cheat a bit on the requirement of uniform motion.The equant was an imaginary point that Ptolemy used to rationalize a planet’s apparently slowing down and speeding up as it wheeled in its epicycles around the deferent. It was possible to establish mathematically that if one were able to view the heavens from the equant, the velocity of a planet would
appear
to be uniform, though from Earth or the eccentric center of the orbit it would appearto vary.
    Ptolemy combined these devices in a complex and highly successful model of heavenly motion. Without removing Earth from its position as unmoving center, his astronomy could, with a surprising degree of accuracy, predict and account for the changing positions of the Sun, the Moon, and the five planets that were known at that time. Fulfilling the hopes of centuries of scholars beforehim, Ptolemy was able to accomplish this feat entirely in terms of circles, spheres, and uniform motion.
    Students and scholars of Tycho Brahe’s and Johannes Kepler’s generations were also steeped from childhood in a worldview that far pre-dated Ptolemy: Nineteen centuries removed from its origin in ancient Greece, Aristotelian philosophy and cosmology still had an enormously strong hold onthe thinking of scholarly and religious Europe. This worldview held that everything below the orbit of the Moon was subject to change, degradation, and decay, while the heavenly spheres beyond the Moon were a realm of unvarying, eternal perfection. Both experience and observation gave weight to these ideas. Before the telescope, there was little evidence to challenge the perfect immutability of theheavenly spheres. Nor was it possible to deny that things were different on Earth.
    This dichotomy had entered the thinking of European Latin-speaking scholars when the first Latin translation of Aristotle appeared in the twelfth century. These men knew nothing of Ptolemy, though the heritage from his astronomy was still flourishing in Islamic parts of the world. They came to revere Aristotle,instead, as the final authority on science and cosmology, and Aristotle’s cosmology, filtered through the understanding of these scholars, merged with medieval Judeo-Christian thought. Somewhat later, when