The Case for Mars by Robert Zubrin Read Free Book Online Page A

treasure trove of Brahe’s observations and that he succeed Brahe as Imperial Mathematician. Kepler now had the ammunition needed to undertake his assault on Mars in earnest.
    Since the time of Aristotle, astronomers had simply assumed that the planets moved in uniform circular orbits because, as Aristotle himself argued, the circle was a perfect form, and only circular motions could come back on themselves and ensure an eternal motion. Try as he might, though, Kepler simply could not get any sort of circular orbit to match Brahe’s observations. True, he could have invoked epicycles, but this Kepler refused to do. Ad-hoc systems of epicycles were irrational—and there had to be a rational answer. But if not circular orbits, what could it be? It took eight years of intense intellectual effort for Kepler to discover what Tycho’s observations of Mars revealed: Mars traveled in an elliptical orbit, with the Sun as one focus of the ellipse. We now know that Mars’ orbit is the most elliptical of all the planets, except Pluto, which was not discovered till the twentieth century, and therefore presented the acid test for any astronomical theory. Indeed, if Mars’ orbit had been circular, the Aristarchus/Copernicus theory would probably have passed muster without anyone looking much deeper.
    Kepler published the results of his labors in 1609 in a work entitled, in full, A New Astronomy Based on Causations or a Celestial Physics Derived from Investigations of the Motions of Mars Founded on the Observations of the Noble Tycho Brahe. Unlike many previous astronomers and philosophers, Kepler declared that this new astronomy was not simply a mathematical construct that reproduced the motions of the heavens. It was, instead, a treatise on the “true reality” of the heavens, an epic work that overthrew two thousand years of dogma and replaced it with an astronomy based on causes. In it he laid out what are now known as Kepler’s first two Laws of Planetary Motion; that the planets move in elliptical orbits with the Sun at one focus, and that the radius vector from the Sun to the planet sweeps out equal areas in equal times. These laws are correct and are found today in all textbooks on astrodynamics. Equally important, however, was what strictly speaking can be called Kepler’s incorrect hypothesis: that the planets were pulled by a “magnetic” force emanating from the Sun, spreading out from it “in the manner of sunlight.” When his opponents accused him of mixing physics with astronomy, Kepler replied, “I believe that both sciences are so closely bound thatneither can achieve perfection without the other.” In other words, Kepler did not describe a model of the universe whose geometry was merely appealing— heas investigating a universe whose causal relationships could be understood in terms of nature knowable to man. In so doing, Kepler catapulted the status of humanity in the universe. Though no longer residing at the center of the cosmos, humanity, Kepler showed, could comprehend it. Therefore, as Kepler wrote to Galileo in the quote that leads this chapter, not only was the universe within man’s intellectual reach, it was, in principle, within physical reach as well.
    Ten years of further study followed, until Kepler was able to publish his masterpiece, The Harmony of the World. Here he laid out his final great discovery, the Third Law of Planetary Motion; that the square of the periods of revolution of the planets is proportional to the cube of their distances from the Sun. Once you have this law, it is a relatively simple matter to derive mathematically what is now known as Newton’s Law of Universal Gravitation. Newton’s laws are the basis of what is known as classical physics, the powerful new body of scientific knowledge that made possible the Industrial Revolution in the eighteenth and nineteenth centuries. With Kepler’s study of the planet Mars, the Dark Ages came to an end, and the scientific and