Void Points, Rosettes, and a Brief History of Planetary Astronomy
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hysics in Perspective
Void Points, Rosettes, and a Brief History of Planetary Astronomy Peter Kosso* Almost all models of planetary orbits, from Aristotle through Newton, include void points, empty points in space that have an essential role in defining the orbit. By highlighting the role of these void points, as well as the rosette pattern of the orbit that often results, I bring out different features in the history of planetary astronomy and place a different emphasis on its revolutionary changes, different from those rendered in terms of epicycles or the location of the earth.
Key words: Aristotle; Eudoxus; Vitruvius; Apollonius; Hipparchus; Ptolemy; Nicolaus Copernicus; Georg Peurbach; Georg Rheticus; Tycho Brahe; Johannes Kepler; Isaac Newton; planetary orbits; epicycles; void points; rosettes; history of astronomy.
Introduction There is no shortage of brief histories of planetary astronomy. Textbooks in astronomy offer a survey from the geocentric Aristotelian model to the heliocentric system of elliptical, Keplerian orbits. There often is implicit disapproval of the extravagance of epicycles, and a sense of triumph when things come neatly together under the influence of universal gravitation. But as in the history of any aspect of the human past, what we get out of the narrative depends in part on what historians decided to put in. Our picture of the past depends on choices made regarding which features to highlight and which events to emphasize. The narrative does not structure itself, so it is important to make explicit the descriptive choices the narrator made. The brief history of planetary astronomy I present below will be guided by an emphasis different from most of the others. This is a bit like staining a specimen differently to reveal new features. It can show different aspects of change and credit the revolutionary events to different contributions. Failure to acknowledge the use of the stain, with a clear indication of what it bonds to, risks producing Whiggish history in which the description of the past is implicitly influenced by our * Peter Kosso teaches philosophy of science at Northern Arizona University in Flagstaff, Arizona.
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modern, tacit sense of what is important. Better to be explicit, since the key to objectivity, in describing the past or in describing nature, is to admit the influence of one’s perspective and to reveal the details of that influence. My brief history of planetary astronomy will be in terms of the concepts of void points and rosettes, highlighting these to describe features of planetary orbits. I will use these concepts to describe changes from one model to the next, from Aristotle through Newton. They also will show features that remain the same from one model to the next, or that disappear only to reappear in later models. This is not to say that void points or rosettes were of concern to, or even noticed by early astronomers. My goal is to see when they did become a concern, and why. This requires highlighting their appearanc
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