Peter Barker, “Constructing Copernicus,” Perspectives on Science 10, no. 2 (Summer 2002).

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Notes

• 208 – “This paper . . . examines Kepler’s unification of physics and astronomy. As an organizing theme, I describe the extent to which the work of Kepler led to the appearance of the form of Copernicanism that we accept today. In the half century before Kepler’s career began, the understanding of Copernicus and his work was significantly different from the modern one. In successive sections I consider the modern conception of Kepler’s contribution to Copernicanism, the most influential sixteenth century view of Copernicus’s work and its sequel, Kepler’s work from the viewpoint of this tradition, and finally the historical origins of the modern view of Copernicanism.”

• “Heliocentrism is the most important single idea that we associate with Copernicus. Accompanying this, although seldom expressed explicitly, is an ontology in which planets are autonomous entities that travel freely through space. . . . The causes of this motion are also important in astronomy, because variation in the causes will produce different orbits.”

• 208-9 – “While it is usual to attribute most of these innovations to Copernicus, a small number are specifically reserved for Kepler. In particular Kepler is credited with the recognition that orbits are ellipses and with the introduction of the law governing the motion of planets around the ellipse (the second law). He is supposed to have been led to these laws by the accurate observations he inherited from Tycho Brahe. Kepler is also generally credited with one of the first serious attempts to establish that these motions are caused by the sun.”

• 209 – “Kepler, at least, expressed a desire to compare theretical predictions with astronomical observations in a very modern sounding way. But the extent to which the work of Kepler led to the appearance of the form of Copernicanism that we accept today has not been generally recognized. In the half century before Kepler’s first publications, the understanding of Copernicus and his work was significantly different from the modern one.”

• 210 – “Copernicus’s work in astronomy indicates several ambitions. He clearly wanted to meet the Averroist challenge to provide a physically significant astronomy, that is an astronomy based on causes, while preserving the precise predictions available in Ptolemaic astronomy. At the same time, he wanted to remove various obstacles to the acceptance of technical devices like those used in Ptolemaic astronomy.”

• 211 – “The last and most important point to understand, however, is that the motions considered by Copernicus are not motions in the same sense as modern astronomy. Copernicus did not consider the orbits of planets, for the simple reason that the concept was not yet available to him. He saw his task, like all astronomers since Ptolemy, as one of calculating the angular position of a planet from the viewpoint of a particular observer on the earth.”

• 212 – “Copernicus’s work was not seen as undermining the account of the heavens in terms of a single celestial substance which carried the planets as it moved, or as undermining hte more detailed conception of the substance as divided into clusters of orbs. Some writers, as a matter of fact, supplied orb constructions for the new combinations of circles introduced by Copernicus.”

• 213 – “Experimentation with different, equivalent, combinations of circles led several different researchers to investigate what is today called a Tychnoic system. In these systems the earth is stationary at the center of the world, the moon goes around the earth, the sun goes around the earth, and all the other planets go around the sun. It was the introduction of this system, in the form developed by Tycho Brahe, that led to the general abandonment of Aristotle’s account of the heavens. Observations of comets played an important role in this development, and also the subsequent development of the concept of a planetary orbit.”

 

• 215 – “Tycho, then, accepted Copernicus’s goal of constructing an astronomy consistent with physics, while rejecting Copernicus’s own proposals. Why was this?

 

• 216 – “The Lutheran viewpoint, that the physical world operates according to a single providentially designed pattern, helps to explain why Tycho was concerned to make astronomy consistent with physics, although in the process he undermined the conventional physics of the heavens.”

 

• “. . . Tycho felt entitled to make new and radical claims about the structure of the world as a whole, which went beyond anything in the classical sources and contradicted both Aristotle and Ptolemy. If the physical world is ordered according to a providential plan, then its parts must fit together into a single scheme. The principles of astronomy must be consistent with the principles of physics. As a Lutheran intent on the religiously sanctioned task of discovering the providential plan, Tycho’s religious beliefs entitle him to an opinion on the structure of the world as a whole, even if it is a surprising one from the viewpoint of traditional scholarship.”

 

• “There he [Kepler] learned not only the Wittenberg approach to astronomy, but Meastlin’s own views on Ptolemy and Copernicus. He also heard about Tycho’s research program, his rejection of solid orbs and his new system of the world. But most important, he absorbed the ideal of Lutheran natural philosophy developed by Melanchthon and his followers, and especially their view that divine providence gave a causal order to the natural world.”
• 218 – “The Sacred Mystery of the Cosmos in 1596 was the first sustained defense of heliocentrism since the publication of Copernicus’s own work in 1543, and Rheticus’s First Account in 1540. . . . Kepler had demonstrated that Copernicanism was consistent with Christianity, indeed that for a Christian who believed in an intelligible providential plan, heliocentrism was superior to the Aristotelian or Ptolemaic order of the world.”
• “The demonstration in the Sacred Mystery established the number and order of the planets, but left many details of planetary motion to be worked out. It was also not clear whether Kepler’s argument showed that copernicanism was superior to the Tychonic system.”
• “And he also established that the physical or true sun should be used as the basis of calculation for planetary positions, where Copernicus had used a constructed point, the mean Sun.”
• 218-9 – “Kepler’s own positive account rejects the vicarious hypothesis. It begins not only with heliocentrism, but with the assumption that the sun plays a causal role in the motion of the planet. A second background assumption specifies a relationship between the distance of the planet from the sun and its velocity, a principle with a long history.”
• 220-21 – “Copernicus’s position clearly requires a new physics but does not provide it. He also fails to give the physical sun a significant role and lacks the concept of an orbit. Kepler is the first ‘Copernican’ to reject celestial spheres. For him the sun plays a causal role in planetary motion, and these motions are described by means of physically real orbits. The differences between Kepler and Copernicus are sufficiently striking that under other circumstances Kepler’s views might have been presented as a new position in astronomy, and not simply a development of Copernicus.”
• 221 – “Kepler needed to escape from the shadow of Tycho . . .. It was therefore natural to ally himself with Copernicus . . .. But in defending his innovations, Kepler provided us with a very different Copernicus from either the original or the Wittenberg version, and thereby provided most of the elements of the modern conception of Copernicanism.”
• 222 – It might well be said that Newton provided the ultimate vindication of the Copernican system only after elaborating universal gravitation. On the basis of this law a small object like the earth must move around a large object like the sun, rather than vice versa. . . . With Newton, then, Copernicanism becomes the thesis that the planets, including the earth, are bound to the sun by gravitational forces which oblige them to move on elliptical paths through free space.”