The Copernican Revolution

Rob Iliffe

8 May 2020

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.

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.

Nicolaus Copernicus (1473-1543)

De RevolutionibusOrbium Coelestium(1543).

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.

1543 De Revolutionibus

annotations by Wittenberg astronomers Erasmus Reinhold and Paul Wittich

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’.

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.

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).

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.

Leonard Digges,

geocentric cosmos based on Apian’sdepiction –from Digges’s almanac,‘ProgosticationEverlastynge’ (1576)

Thomas Digges,

‘A Perfit Description of the CoelestiallOrbes according to the most ancient doctrine of the Pythagoreans –

added to his father’s [1576]

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.

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.

SN1572

Hubble photograph of remnant of Tycho’ssupernova

Johannes Kepler (1571-1630)

MysteriumCosmographicum (1596)

Astronomia Nova (1609)

Harmonices Mundi(1619)

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.

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.

Bibliography