Historical context

• We are now moving about 200 years from the Middle Ages to the Renaissance in the 15th & 16th centuries. • Major changes in this period: ! Voyages of discovery 5.Copernicus ! Protestant Reformation and weakened Roman Church ! Invention of movable type & printing press ! Rise of a middle class ! Humanist movement in scholarship and education • The problem of celestial motions is one of the outstanding problems in natural philosophy.

Copernicus Nicholas Copernicus (1473-1543) • Copernicus was born in Poland. • Got a degree from the Univ of Crakow. • Then studied medicine and canon law in Padua & Bologna. • Returned to Poland in 1503, and worked as a church administrator under his uncle, a Bishop. • He was clearly fascinated with astronomy, but felt Ptolemy’s eccentrics and especially the equant, violated Aristotle’s principles of motion. • Thought that he could return to a more Aristotelean solution by putting the Sun at the center of the solar system. • Wrote a short description of his ideas in the Commentariolus ca. 1514 which circulated in manuscript form.

Copernicus’ helio-centric model Copernicus, cont’d • Astronomy professor Georg Rheticus from Wittenberg visited and persuaded Copernicus to publish his ideas. • Copernicus’ objection to the Ptolemaic system was that it had got away from the fundamental principles of • Finally, in 1543, Copernicus published On the Aristotle’s dynamics, and not that it was inaccurate in its Revolutions of Heavenly Bodies, a proposal that the Sun predictions of celestial events. is at the center of the solar system, rather than the Earth. • However it turned out that Copernicus still required epicycles and eccentric motion to account for things like the retrograde motion of the planets, and he needed 46 circular motions for his model. • So, it was still quite complicated to do astronomical calculations with Copernicus’ model, but much more simple to calculate with than the Ptolemaic model that required 70 circular motions.

Copy of De Revolutionibus in Vilnius Library

Publication history

•Commentariolus (Little Commentary) circulated in MS form after 1514

•Narratio Prima (First Account) by Georg Joachim Rheticus, 1540

•De Revolutionibus Orbium Coelestium (On the Revolution of the Heavenly Spheres by Copernicus (foreword by Osiander), 1543 Copernican model (greatly exaggerated) Copernicus: summary

• But Copernicus’ model did require many fewer circles ---Earth’s orbit than the Ptolemaic model, and so it was easier to calculate with. • It was used for calculating the astronomical tables that were the basis of the Gregorian calendar. ---Center of Earth’s orbit • But the Sun-centered model was largely regarded as an algorithm for calculation, and the Ptolemaic model was Sun considered the actual description of reality--it “made sense.” (How could birds fly, for example, if the Earth were moving so fast round the Sun?) There was no theory of mechanics to explain how the Earth & other planets could go round the Sun. N.B. Copernicus still required epicycles and eccentric motion. • The Aristotelean idea of natural motion in the heavens was what “made sense.“