While Galileo is famous as the father of modern astronomy, the German astronomer Johannes Kepler played an equally impressive role in proving that the sun, and not Earth, lay at the centre of the cosmos.
In 1609 the accepted view, and the one endorsed by the church, was that Earth was at the centre of the universe and did not revolve, and that the moon, sun and planets moved around it. The Greeks had originated this view of the cosmos and Ptolemy had formalised it as an astronomical system in the second century A.D. While Ptolemaic astronomy was not simple—it employed a complex system of circles and epicycles to imitate the motion of the planets—it had been accepted as truth for almost a millennium and a half.
In the middle of the 16th century Copernicus advanced the radical theory that the sun was the centre of the universe, with Earth and the other planets rotating around it. Copernicus' treatise on heliocentrism, On the Revolutions of Heavenly Spheres, was published in 1543. Kepler discovered it when he studied at the University of Tübingen and he was moved by Copernicus's views. But Kepler was an exception: most people were unhappy with the concept of a heliocentric universe. First of all, Copernicus' ideas were virulently opposed to the teachings of the Roman Catholic Church. In addition, many astronomers contended that Copernicus's heliocentric astronomy was not much more accurate than Ptolemaic astronomy: Ptolemy had predicted positions of Mars approximately every 32 years that were in error by about 5 degrees in longitude. Copernicus' system was off by about 4 degrees longitudinally. One historian has called this "the great Martian catastrophe"—a problem that observers such as Danish astronomer Tycho Brahe knew about, but that Kepler would solve.
In 1601, Kepler created an initial model of the motion of Mars in which he assumed that Mars revolved around the sun in a circular orbit. This model did not conform to the observations of Tycho Brahe, whose work Kepler continued, so Kepler redrew the orbit as more elliptical. "There is a myth that Kepler, fitting a curve through Tycho Brahe's records of Mars, discovered that planetary orbits are elliptical," the historian says. "The fact is that Tycho's observations showed that the orbit was not a circle, but the choice of an ellipse was largely theoretical." It was one of those intellectual leaps that would change the course of science. Kepler found that not only did an elliptical orbit with the sun explain the movement of Mars, but also of the other planets. In fact, as the historian points out, in the Astronomia Nova published in 1609, the typeface suddenly becomes larger to account for the significance as Kepler explains the motion of Mars and presents the first two of his laws describing planetary motion.
The first law states that the planets travel in elliptical orbits around the sun and describes the sun's position as the focal point in that ellipse. The second law states that an imaginary line connecting a planet to the sun will sweep out a region of equal size in a given time period, wherever in the orbit that time period falls. But the underlying physical reason for the planetary motion eluded Kepler, who thought a sort of magnetism was responsible. That puzzle would have to wait for another revolutionary thinker, Isaac Newton, whose law of gravity appeared on the scientific stage and explained orbital behaviour eight decades later.
(a) Kepler has been unfairly eclipsed by Galileo in the history of astronomical science.
(b) Kepler's most significant discovery regards the elliptical and not the circular orbit of the planets.
(c) A survey of the discoveries of Kepler reveals his contribution as a revolutionary thinker on the level of Copernicus, Galileo and Newton.
(d) Kepler had the privilege of completing the work begun by Tycho Brahe.
(e) Kepler solved a fundamental problem astronomers had with Copernicus's discoveries.
(a) More planets revolve around the sun than has heretofore been known.
(b) New mathematical formulas predict the position of Mars with perfect accuracy.
(c) Highly precise astronomical instruments find that planetary orbits are circular after all.
(d) Physicists continue to refine Newton's basic law of gravity.
(e) Minute stars are discovered to revolve around the sun.
(a) "an exception"
(b) "did not conform"
(c) "a myth"
(d) "intellectual leap"
(e) "largely theoretical"
(a) intimate that a historical opportunity was given to Kepler's investigations
(b) highlight an unfortunate circumstance of Copernicus' theories
(c) reinforce the gravity of Ptolemy's erroneous Earth-centric beliefs
(d) emphasise the hit-and-miss nature of scientific discovery
(e) point to the fact that the Church exploited astronomy's error
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