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The Copernican Revolution
Rob Iliffe
8 May 2020
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End of Aristotle?
• Internal contradictions within the Aristotelian system caused increasing problems for natural philosophers in the C16.
• Rival philosophies based on views of Plato (‘Neo-Platonism’) and other heterodox (e.g. ‘Hermetic’) sources were published.
• Celestial perfection was brought into question by observations of supernovae in 1572 and 1604,
• and by the determination that the path of 1577 comet was superlunary.
• Crucially, in the early Seventeenth Century, it would become acceptable to use artificial devices and techniques such as instruments, lenses, and experiments to understand Nature.
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Ptolemaic System
• Claudius Ptolemy’s C2 Mathematical Compilation (later known as Almagest) was major exposition of geocentric astronomy until C16, and basis of medieval/Renaissance horoscopes and almanacs.
• Existed uneasily alongside Aristotle’s doctrine of homocentric spheres.
• Undergraduate students in medieval universities would learn basic aspects of astronomy from abridgements of Sacrobosco’s de sphaera;
• More serious students studied the Almagest itself with increasingly sophisticated commentaries.
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Nicolaus Copernicus (1473-1543)
De RevolutionibusOrbium Coelestium(1543).
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Copernican heliocentrism
• In De Revolutionibus, Copernicus argued that the Earth was a planet with three motions (daily rotation, annual revolution, and annual tilting of its axis)
• and his account explained better the retrograde motion of planets (a visual effect of annual terrestrial orbit).
• Copernicus claimed his view was simpler than that of his predecessors though in fact he needed to retain a number of Ptolemaic devices to account for non-circular orbits.
• Only 10 people before 1600 agreed that the Copernican theory was probable, preferable or true…
• The most important being Johannes Kepler and Galileo Galilei.
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1543 De Revolutionibus
annotations by Wittenberg astronomers Erasmus Reinhold and Paul Wittich
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Osiander’s ‘Preface’
• The ‘Preface’ to De revolutionibus (by his disciple Andreas Osiander) claimed that Copernicus’ system was only a technical device for making better predictions of heavenly motions…
• and that the correct hierarchy of the disciplines had been preserved -
• i.e. the book did not meddle with the physical reality of the heavens.
• This ‘Preface’ was influential throughout rest of C16th, prompting astronomers to treat Copernicus’s work as a useful ‘fiction’ or ‘instrument’.
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Critiques of heliocentrism
• 1. That heliocentrism disobeyed Aristotelian dictum that each body only had one proper motion.
• 2. That it conflicted with common sense and the daily experience of the sun rising and setting.
• 3. That, given the angular velocity of a rotating Earth, it was physically absurd, since objects on the face of the Earth would fly off.
• 4. That it was in conflict with Scripture, e.g. Joshua 10:12-13 – ‘Sun stand thou still at Gibeon’
• 5. That it upset the order of the disciplines, in which natural philosophy was superior to mathematics.
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De Revolutionibus
• However, Copernicus clearly argued that his theory was not merely a technical device but explained the real motions of heavenly bodies.
• Copernicus argued that heliocentrism had classical (pre-Aristotelian) antecedents,
• And offered a physical explanation of why objects did not fly off the earth’s surface
• and did not conflict with Scripture (which was literally geocentric).
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Copernican Innovations
• Copernicus’s work was not just innovative in terms of its content:
• 1. By telling his readers that they could not pass comment on his work unless they were mathematically trained,
• he argued that technically proficient astronomers had the right and capacity to make claims about the physical world, thus extending the bounds of mathematics into the domain of natural philosophy;
• 2. Copernicus thus provided template for a new kind of hybrid professional identity;
• 3. He and his followers made it possible to argue that innovations in natural philosophy had to take place outside the university.
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Leonard Digges,
geocentric cosmos based on Apian’sdepiction –from Digges’s almanac,‘ProgosticationEverlastynge’ (1576)
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Thomas Digges,
‘A Perfit Description of the CoelestiallOrbes according to the most ancient doctrine of the Pythagoreans –
added to his father’s [1576]
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Infinitely large universes
• Aristotelian division between the terrestrial sphere and the more perfect celestial sphere broke down with the appearance of a “New star’ (Supernova) in 1572
• And determination that comet of 1577 lay outside sphere of the Moon.
• These findings created opportunity for astronomers and natural philosophers to posit new theories about constitution of the heavens
• And even to posit an infinitely large cosmos.
• This view had previously been seen to be dangerous, because it threatened to equate God with the universe itself but it was promoted in late C16 by Giordano Bruno.
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Tycho Brahe (1546-1601)
• Born into aristocratic family in what was then Denmark.
• Went to Univ. Copenhagen at age 13, where he studied astronomy.
• In 1572 he made extensive studies of the new star, which he proved to be a distant phenomenon and not sublunary.
• Danish king gave him the island of Hven where he built his own observatory, alchemical laboratory and printing press.
• Remained a profound anti-Copernican, largely for religious reasons, and devised his own ‘geoheliocentric’ system.
• Made Imperial Mathematician at Prague in 1598, where he was joined by his ‘assistant’ Johannes Kepler in 1600.
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SN1572
Hubble photograph of remnant of Tycho’ssupernova
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Johannes Kepler (1571-1630)
MysteriumCosmographicum (1596)
Astronomia Nova (1609)
Harmonices Mundi(1619)
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Kepler, Copernican
• Kepler trained at Tuebingen under Copernican mathematician astronomer Michael Maestlin
• Maestlin knew Copernican ‘Preface’ was by Osiander and told Kepler.
• Kepler was convinced Pythagorean who believed that Copernicanism had to be true because of mathematical reasons –
• In Mysterium Cosmographicum (1696) Kepler proposed that distances between planets could be explained by appealing to polyhedral forms.
• Also argued that biblical geocentric passages could be explained away by appeal to ‘accommodationism’
• Also worked on a text, the ‘Somnium’, which could explain heliocentrism through form of dream-like trip to the moon.
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From Circles to Ellipses
• In 1601 Kepler, now at Prague, inherited the astonishingly accurate (pre-telescopic) measurements of the Martian orbit made by Tycho
• He noticed that Tycho’s data showed that the orbit was not circular
• At first he tried a number of different ovoidal (egg) forms to fit the data but then determined that it was an ellipse.
• He generalised this idea to all the known planets
• This, his first ‘law’ (planets travel in ellipses), was published with his second ‘law’ in Astronomia Nova in 1609.
• Second law: with respect to either of the two foci of their elliptical orbit, planets sweep out equal areas in equal times.
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Bibliography