Western Civilization_ Volume B_ 1300 to 1815 - Jackson J. Spielvogel [218]
Hence I feel no shame in asserting that this whole region engirdled by the moon, and the center of the earth, traverse this grand circle amid the rest of the planets in an annual revolution around the sun. Near the sun is the center of the universe. Moreover, since the sun remains stationary, whatever appears as a motion of the sun is really due rather to the motion of the earth.
What major new ideas did Copernicus discuss in this excerpt? What was the source of these ideas? Why might one say that European astronomers finally destroyed the Middle Ages?
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Kepler published his first two laws of planetary motion in 1609. Although at Tbingen he had accepted Copernicus’s heliocentric ideas, in his first law he rejected Copernicus by showing that the orbits of the planets around the sun were not circular but elliptical, with the sun at one focus of the ellipse rather than at the center. In his second law, he demonstrated that the speed of a planet is greater when it is closer to the sun and decreases as its distance from the sun increases. This proposition destroyed a fundamental Aristotelian tenet that Copernicus had shared—that the motion of the planets was steady and unchanging. Published ten years later, Kepler’s third law established that the square of a planet’s period of revolution is proportional to the cube of its average distance from the sun. In other words, planets with larger orbits revolve at a slower average velocity than those with smaller orbits.
Kepler’s three laws effectively eliminated the idea of uniform circular motion as well as the idea of crystalline spheres revolving in circular orbits. The basic structure of the traditional Ptolemaic system had been disproved, and people had been freed to think in new ways about the actual paths of planets revolving around the sun in elliptical orbits. By the end of Kepler’s life, the Ptolemaics ystem was rapidly losing ground to the new ideas. Important questions remained unanswered, however: What were the planets made of? And how could motion in the universe be explained? It was an Italian scientist who achieved the next important breakthrough to a new cosmology by answering the first question and making important strides toward answering the second.
Johannes Kepler. Abandoning theology in favor of mathematics and astrology, Kepler became a key figure in the rise of the new astronomy. Building on Tycho Brahe’s vast store of astronomical data, Kepler discovered the three laws of planetary motion that both confirmed and modified the Copernican theory. They also eliminated the Ptolemaic-Aristotelian ideas of uniform circular motion and crystalline spheres moving in circular orbits. This portrait was done by an unknown painter three years before Kepler’s death.
Musée de l’Oeuvre Notre Dame, Strasbourg//© Imagno/Austrian Archives/Getty Images
Galileo
Galileo Galilei (1564–1642) taught mathematics, first at Pisa and later at Padua, one of the most prestigious universities in Europe. Galileo was the first European to make systematic observations of the heavens by means of a telescope, thereby inaugurating a new age in astronomy. He had heard of a Flemish lens grinder who had created a “spyglass” that magnified objects seen at a distance and soon constructed his own after reading about it. Instead of peering at terrestrial objects, Galileo turned his telescope to the skies and made a remarkable series of discoveries: mountains and craters on the moon, four moons revolving around Jupiter, the phases of Venus, and sunspots. Galileo’s observations demolished yet another aspect of the traditional cosmology in that the universe seemed to be composed of material substance similar to that of the earth rather than ethereal or perfect and unchanging substance.