137 - Arthur I. Miller [38]
Given his poor health, interest in religion, and excellent record in the elementary Latin school, the obvious choice of career was to join the clergy. In 1589 he entered the theology school at the University of Tübingen. The professor of mathematics and astronomy there, Michael Maestlin, invited him to join his private study group.
Copernicus’s sun-centered universe
In his public lectures Maestlin taught Ptolemy’s model of the universe with the sun and stars circling the earth, which agreed with Christian theology. But in private he was intrigued by the world-picture proposed by the Polish priest Nicholas Copernicus in his book of 1543, De Revolutionibus (On the Revolutions), which put the sun, not the earth, at the center of the planetary orbits.
In Copernicus’s model, the six planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) make circular orbits around the sun, all enclosed by the sphere of the fixed stars. But Copernicus was perfectly aware that planets do not move in this way. A planet moving from west to east might cut back to the west, then resume an easterly orbit (known as retrograde motion). To explain these more complicated motions, Copernicus set planets moving on circles whose centers were on the surfaces of other circles that were also in circular motion, making the motions of planets the sum total of many circular motions. When more precise observations resulted in one of these planetary systems falling out of line, he added yet more circles. Thus he was able to explain the observed motion of the planets.
Copernicus’s 1543 model of the universe. The sun (sol) is at the center with the six planets moving around in circular orbits. All this occurs within the seventh sphere—the sphere of the fixed stars (Stellarum fixarum). (Copernicus, De Revolutionibus [1543].)
Church authorities condemned Copernicus’s system as heresy, in that even though his system offered the best available mathematical basis for a twelve-month calendar, it did not place the earth at the center of the universe. (By “universe,” Copernicus and his contemporaries meant what we now know as the solar system.) Church scientists expurgated De Revolutionibus, declaring that assertions made with certainty were in fact merely hypothetical. But Maestlin believed otherwise: that this was the way things actually were.
Kepler, too, read Copernicus’s book. Two passages fired his imagination. In one Copernicus wrote, “In the middle of all sits the Sun enthroned. In his beautiful temple could we place this luminary in any better position from which he can illuminate the whole at once? He is rightly called the Lamp, the Mind, the Ruler of the Universe; Hermes Trismegistus names him the Visible God, Sophocles’ Electra calls him the All-seeing. So the sun sits as upon a royal throne ruling his children, the planets which circle around him.”
In the other Copernicus wrote of his model of the sun-centered universe: “We find in this arrangement a marvelous symmetry of the world and a harmony in the relationship of the motion and size of the orbits, such as one cannot find elsewhere.” These words struck Kepler like a bolt of lightning. He was gripped by the notions of order and harmony.
More than two hundred years were to pass before scientists were able to give incontrovertible proof that the earth circled the sun by measuring stellar parallax—the change in a star’s position caused by the earth’s movement around the sun. The only proof Copernicus had was his sense of aesthetics and