The Celestial Spheres

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The Celestial Spheres

The Queen of the Sciences. The desire to find general laws that governed the universe provided the rationale for the wide development of one side of medieval science. In the Middle Ages, as well as during the Renaissance, scholars believed that God revealed his handiwork in nature, so its laws were divine truths and understanding them would lead to a greater understanding of God. Believing that God is located in the heavens, medieval scholars placed greater importance on astronomy and Christian astrology than on other sciences. From the eleventh century on, many Churchmen thought and wrote about the workings of the heavens, making astronomy the Queen of the Sciences.

The Celestial Spheres. Medieval astronomy differed little from its ancient predecessor. Based on the concept of an earth-centered universe that had been most clearly enunciated by Alexandrine astronomer Claudius Ptolemy in the first century C.E., the medieval model of the heavens served one purpose alone: to explain God’s creation. But, as in all medieval science, they had to reconcile their Christian view of the universe with Aristotle’s ideas about it. The Ptolemaic and Aristotelian cosmologies are complementary in many ways, but they approach astronomy differently, and neither is a perfect reflection of what people see in the heavens.

Aristotle’s universe. The main difference between Aristotelian and Ptolemaic astronomy was that Aristotle devised a physical theory to account for the motion of the planets, and Ptolemy provided a mathematical one. Aristotle’s physical theory posited many crystalline spheres, centered on the earth and nested one within another like the layers of an onion. Each sphere contains a specific substance or body and communicates motions to its neighbors. The earth is surrounded by spheres of water, air, and fire; seven spheres for the sun, the moon, and the five known planets (Mercury, Venus, Mars, Jupiter, and Saturn); and one sphere for the fixed stars. The primum movens (first mover), the ultimate spirit or intelligence, activates the primum mobile (first moved), which in turn sets the outer sphere in motion, and so on through the inner spheres. The idea of a primum movens—the Unmoved Mover—was attractive to Christian philosophers because God could easily be placed in that role.

Ptolemy’s Universe. Ptolemy’s mathematical explanation agreed with Aristotle’s ordering of the planets but omitted the spheres of elements around the earth and minimized the need for a primum movens. Ptolemy’s universe was described by the mathematical relationships of the distances between the planets and their relative rates of motion. Ptolemaic astronomy—as transmitted through Arabic astronomers—had taken on an explanatory power as well. The idea of mathematical causation has an ancient pedigree, going back to the ancient Greek thinkers Plato and Pythagoras, who believed in the ultimate reality of numbers. If they were real, they could cause things. Yet, Ptolemy’s system was primarily descriptive and predictive, rather than explanatory, like Aristotle’s.

Planetary Motion. From Ptolemy medieval astronomers learned the ideas of the epicycle and the deferent. These two mathematical concepts were used to explain why—although Ptolemy, like Aristotle, was sure that all motion in the heavens was circular in shape and uniform in velocity—the planets appeared to wander among the stars in decidedly noncircular paths and at nonuniform velocities. In fact, against the backdrop of the fixed stars, some planets, such as Mars, appeared to stop and reverse direction. The epicycle-deferent system solved that apparent problem. The deferent, according to Ptolemy, was a circular path centered on the earth. The epicycle was a smaller circle in which a planet moved at a uniform speed. The center

of the epicycle was located on the deferent and proceeded to move around it at a uniform rate. By creating the notion of two separate and uniform circular motions—and in some later refinements even three—the Ptolemaic system provided an accurate way to chart and predict the position of the planets.

Astrology . Medieval astronomers were also astrologers. While the positions of the planets are not of much use for calendar calculations, the known planets, and the stars, nonetheless held great meaning for medieval Europeans, who considered astronomy (the “naming” of the heavens) and astrology (the “laws” of the heavens) as two parts of the same science. This tradition was also inherited from antiquity; for not only did Ptolemy write his Almagest (The Great Mathematical Synthesis, a name acquired from the Arabic versions) on the motions of the planets, he also composed the Tetrabiblos (Four Books) of astrology, which set forth the basic rules of what the heavens signified for human activities. His rules about the constellations in which the planets appear to rise and set, as well as the relationships of the planets to each other and the signs of the zodiac, were all set forth in great mathematical detail. So too were the processes to calculate three kinds of horoscopes: “genethliacal” horoscopes that relate to the moment of an individual’s birth and fate; “mundane” (worldly) horoscopes that deal with affairs of state; and “electional” horoscopes that deal with determining the correct time for launching any enterprise.