Free Coming of Age in the Milky Way by Timothy Ferris Page B

scholars who ventured to quantify them did so by assuming that the various orbits and epicycles fit snugly together, like nested Chinese boxes. The Copernican theory, however, precisely stipulated the relative dimensions of the planetary orbits: The maximum apparent separation of the inferior planets Mercury and Venus from the sun yields the relative diameters of their orbits, once we accept that both orbit the sun and not the earth. Since the relative sizes of all the orbits were known, if the actual distance of any one planet could be measured, the distances of all the others would follow. As we will see, this advantage, though purely theoretical in Copernicus’s day, was to be put to splendid use in the eighteenth century, when astronomical technology reached the degree of sophistication required to measure directly the distances of nearby planets.
    The immediate survival of Copernicanism was due less to any compelling evidence in its favor than to the waning fortunes of the Ptolemaic, Aristotelian model. And that, as it happened, wasprompted in large measure by changes in the sky—by the apparition of comets, and, most of all, by the fortuitous appearance of two brilliant
novae
, or “new stars,” during the lifetimes of Tycho, Kepler, and Galileo.
    Integral to Aristotle’s physics was the hypothesis that the stars never change. Aristotle saw the earth as composed of four elements—earth, water, fire, and air—each of which naturally moves in a vertical direction: The tendency of earth and water is to fall, while that of fire and air is to rise. The stars and planets, however, move neither up nor down, but instead wheel across the sky. Aristotle concluded that since objects in the sky do not partake of the vertical motion characteristic of the four terrestrial elements, they must be made of another element altogether. He called this fifth element “aether,” from the Greek word for “eternal,” and invested it with all his considerable reverence for the heavens. Aether, he argued, never ages or changes: “In the whole range of time past,” he writes, in his treatise
On the Heavens
, “so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts.” 15
    Aristotle’s segregation of the universe into two realms—a mutable world below the moon and an eternal, unchanging world above—found a warm welcome among Christian theologians predisposed by Scriptures to think of heaven as incorruptible and the earth as decaying and doomed. The stars, however, having heard neither of Aristotle nor of the Church, persisted in changing, and the more they changed, the worse the cosmology of Aristotle and Ptolemy looked.
    Comets were an old problem for the Aristotelians, since no one could anticipate when they would appear or where they would go once they showed up. * (It was owing to their unpredictability that comets acquired a reputation as heralds of disaster—from theLatin
dis-astra
, “against the stars.”) * Aristotle swept comets under the rug—or under the moon—by dismissing them as atmospheric phenomena. (He did the same with meteors, which is why the study of the weather is known as “meteorology.”)
    But when Tycho Brahe, the greatest observational astronomer of the sixteenth century, studied the bright comet of 1577, he found evidence that Aristotle’s explanation was wrong. He triangulated the comet, by charting its position from night to night and comparing his data with those recorded by astronomers elsewhere in Europe on the same dates. The shift in perspective produced by the differing locations of the observers would have been more than sufficient to show up as a difference in the comet’s position against the background stars, were the comet nearby. Tycho found no such difference. This meant that the comet was well beyond the moon. Yet Aristotle had held that nothing superlunar could change.
    The