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and medicine. The cosmological views of the Later Middle Ages had been built on a synthesis of the ideas of Aristotle, Ptolemy (the greatest astronomer of antiquity, who lived in the second century C.E.), and Christian theology. In the resulting Ptolemaic (tahl-uh-MAY-ik) or geocentric conception, the universe was seen as a series of concentric spheres with a fixed or motionless earth at its center. Composed of the material substances of earth, air, fire, and water, the earth was imperfect and constantly changing. The spheres that surrounded the earth were made of a crystalline, transparent substance and moved in circular orbits around the earth. Circular movement, according to Aristotle, was the most “perfect” kind of motion and hence appropriate for the “perfect” heavenly bodies thought to consist of a nonmaterial, incorruptible “quintessence.” These heavenly bodies, pure orbs of light, were embedded in the moving, concentric spheres, which in 1500 were believed to number ten. Working outward from the earth, eight spheres contained the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and the fixed stars. The ninth sphere imparted to the eighth sphere of the fixed stars its motion, and the tenth sphere was frequently described as the prime mover that moved itself and imparted motion to the other spheres. Beyond the tenth sphere was the Empyrean Heaven—the location of God and all the saved souls. This Christianized Ptolemaic universe, then, was finite. It had a fixed outer boundary in harmony with Christian thought and expectations. God and the saved souls were at one end of the universe, and humans were at the center. They had been given power over the earth, but their real purpose was to achieve salvation.

This conception of the universe, however, did not satisfy professional astronomers, who wished to ascertain the precise paths of the heavenly bodies across the sky. Finding that their observations did not always correspond to the accepted scheme, astronomers tried to “save appearances” by developing an elaborate system of devices. They proposed, for example, that the planetary bodies traveled on epicycles, concentric spheres within spheres, that would enable the paths of the planets to correspond more precisely to observations while adhering to Aristotle’s ideas of circular planetary movement.

Medieval Conception of the Universe. As this sixteenth-century illustration shows, the medieval cosmological view placed the earth at the center of the universe, surrounded by a series of concentric spheres. The earth was imperfect and constantly changing, whereas the heavenly bodies that surrounded it were perfect and incorruptible. Beyond the tenth and final sphere was heaven, where God and all the saved souls were located. (The circles read, from the center outward:

© Image Select/Art Resource, NY

Copernicus

Nicolaus Copernicus (nee-koh-LOW-uss kuh-PURR-nuhkuss) (1473–1543) had studied both mathematics and astronomy first at Krakow in his native Poland and later at the Italian universities of Bologna and Padua. Before he left Italy in 1506, he had become aware of ancient views that contradicted the Ptolemaic, earth-centered conception of the universe. Between 1506 and 1530, he completed the manuscript of his famous book, On the Revolutions of the Heavenly Spheres, but his own timidity and fear of ridicule from fellow astronomers kept him from publishing it until May 1543, shortly before his death.

Copernicus was not an accomplished observational astronomer and relied for his data on the records of his predecessors. But he was a mathematician who felt that Ptolemy’s geocentric system was too complicated and failed to accord with the observed motions of the heavenly bodies. Copernicus hoped that his heliocentric or sun-centered conception would offer a simpler and more accurate explanation.

Copernicus argued that the universe consisted of eight spheres with the sun motionless at the center and the sphere of the fixed stars at rest in the eighth sphere. The planets revolved around the sun in the order of Mercury,