Download Ptolemy and Copernicus: Ancient and Modern Cosmological Models - Prof. Elizabeth Anne Lada and more Study notes Astronomy in PDF only on Docsity! 1 Ptolemy (125 A.D.) • Designed a complete geometrical model of the universe that accurately predicted planetary motions with errors within 50 • Most of the geometric devices and basic foundations of his model did not originate with him but were based on the models of the early Greeks such as Aristotle & Hipparchus • Wrote the Almagest (Greatest) – included the original works & models of Ptolemy – included a compilation of past works of Greeks, especially Hipparchus – 13 volumes Ptolemy’s Model • Earth was spherical & at center of cosmos - GEOCENTRIC • Cosmos is finite in size • Earth has no motions • Sun, Moon Planets exhibit uniform, circular motions – natural motions - no forces • Used devices of eccentrics, epicycles & deferents to explain the observed non-uniform motions of the Sun and planets along the ecliptic & retrograde motion. • Introduced equant to explain the variations in retrograde motions Ptolemy’s Equant • point inside a circle, not at the center, from which motion along the circumference of the circle would appear to be uniform • opposite the circle’s center from the eccentric (the Earth) • nonphysical geometrical device that broke fundamental assumption of uniform circular motion Celestial motions no longer had to be uniform around the centers of circles Ptolemy’s Model Copernicus (1473-1543) • Developed a Heliocentric (Sun centered) model of the cosmos • Why? Ptolemy’s geocentric model lasted for centuries mainly because it accurately predicted celestial motions so there was little reason to discard it • Copernicus studied the works of Aristotle, Pythagoras & Plato • an offshoot of Plato’s philosophy asserted that Sun was godhead of all knowledge • Copernicus objected to equant based on aesthetics - equant not faithful to ideal of uniform motion - makes models to complex Copernicus’ Model • Copernicus worked on his new Heliocentric model for 20 years – Sun was placed at center of cosmos – Earth no longer static, but revolved around Sun once a year & rotated on axis once a day • His work was published in De revolutionibus in the year of his death • De revolutionibus took after the Almagest in outline and basic intention - to explain planetary motions • Even though it took 20 years to develop this model did predict celestial motions any better than Ptolemy’s geocentric model 2 Heliocentric Model of Copernicus • Cosmos finite in size • Assumed no forces for heavenly motions – Physics of Aristotle • Assumed uniform, circular motions – done for aesthetics - followed Aristotle • All heavenly spheres revolve around the sun & the sun is at the center of the cosmos – chosen based on aesthetics and simplicity • The distance from the earth to the sphere of stars is much greater than the distance from the earth to the sun – accounts for lack of observed stellar parallax Heliocentric Model of Copernicus • The daily motion of the heavens relative to the horizon results from the earth’s motion on its axis – aesthetic appeal since only 1 sphere is rotating not many – however, he did not account for the objection that if the earth rotated, objects should be flung from the surface • The apparent motion of the sun relative to the stars results from the annual revolution of the earth around the sun • The planets’ retrograde motion occur from the motion of the earth relative to the other planets – retrograde explained as a natural result of the planet’s revolutions about sun - what we observe is an illusion Retrograde Motion Explained Retrograde Motion Explained • When the earth passes any of the outer planets or when the earth is passed by the inner ones, retrograde motion occurs • Passing is key to retrograde motion in the heliocentric model • Copernicus eliminated epicycles to explain retrograde motion • Eliminated equant - kept uniform circular motion • Needed to account for variations in planetary motion so he was forced to add many smaller circles • Violated Aristotelian physics & did not offer new physical ideas to support his model • Didn’t predict motions any better than Ptolemy’s model Copernican Model Ptolemy
