Geocentric Model of the Universe – Ptolemy (et al.) 5 known planets (excluding Earth) Sun is beyond Venus (note that Venus and Mercury are closer to the Sun than Earth). Stars are beyond planets and not moving wrt each other.
Claudius Ptolemais (~150 BC) • Built on Eudoxus (~408 BC), Aristotle (384-322BC), Hipparchus and others • Almagest (Al Magest), vastly influential • introduced equant (uniform angular movement) • 40 epicycles and deferents • Equants and eccentrics for all planets (including Sun&Moon) • Predicted motions well • Demo 123 Details 4
Ancient Greece: not all geocentricPythagoras (~550BC) • Non-geocentric model • Taught that spheres are the perfect shape • Earth and other celestial bodies were spherical • Revolved around central fire • Counter-Earth (antichthon), blocks view from central fire • Ten spheres (perfect number?) • ‘Music of the spheres’
Ancient Greece: not all geocentricAristarchus of Samos (?310-250BC) • Sun was >20 times farther away than Moon • Heliocentric model • Details not known, secondary sources • Earth rotates daily • Earth revolves around Sun • Stellar parallax not observed because stars are too far away • Model not accepted, too ‘radical’.
Why a geocentric model might be considered preferable • More intuitive, ego-centric, gives man importance • Spinning Earth left would create fast E-W winds • We don’t feel Earth spinning • What keeps Earth spinning? Earth too big to move (no rotation, no revolution) • Aristotle’s ‘natural tendency’ (earth/water fall, air/fire rise) • If stars are too far away for stellar parallax, universe would be too big. • No stellar parallax observed • Stars’ brightness should change with season • Falling objects would take curved paths • Moon would be left behind
Copernicus (1473-1543) • De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orbs"), published 1543 • Wished to eliminate Ptolemy’s equant (violated uniform circular motion, unaesthetic) • Heliocentric model • Earth rotates daily and revolves around Sun annually • Still assumes uniform circular motion • Retains epicycles (48, i.e. 8 more than Ptolemy)
Objections to Copernicus • Predictions not better than before • More epicycles than Ptolemy • Religious objections (not just Catholic church) • Rotational and orbital speed of Earth must be incredibly large (~1600km/h, equator, and 30km/s) • No difference in stellar brightness observed (should be there if distance varies) • No stellar parallaxes observed.
Tycho Brahe (1546-1601) • Danish nobleman • Pro-Aristotle, anti-Copernicus • Lost part of his nose in duel • Most precise pre-telescope observations (1-2’) • Crucial planetary motion data (Mars) • Observations of supernova (1572) and comet (1577), ‘change in the sky’ • Best measurements yet of stellar parallax, found none -> geocentric universe • Moon and Sun revolve around Earth • Planets revolve around Sun
Brahe’s geo(helio)centric universe • Found no stellar parallax, two choices, chose wrong one • Geocentric model • Earth unmovable at center • Sun and Moon revolve around Earth • Other planets revolve around Sun • Epicycles and equants
Johannes Kepler (1571-1630) • Became assistant to Brahe in 1600 • Did not get along with Brahe, competition • Mars data, 1601-05 • ‘listened’ to data • Sought ‘celestial harmonies’ • Published his 3 laws in 1609 (Astronomia Nova, Law 1 and 2) and 1618 (Harmonice Mundi, Law 3)
Kepler’s laws of planetary motion, First Law The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.
Demo • AF1305.html • AF1307.html
Kepler’s laws of planetary motion, Second Law The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.Link
Demo • AFc1307.html
Kepler’s laws of planetary motion, Third Law The squares of the planets’ periods of revolution are proportional to the cubes of the semimajor axes of their orbits. P2 ~ a3
Galileo Galilei (1564-1642) • Mathematician, observer, experimenter • Anti-Aristotelian • Did not invent, but made his own telescopes (1609 …3x – 30x) • Cannonballs, Pisa • Craters and mountains on Moon -> perfect shape? • Moons of Jupiter • Phases of Venus (Ptolemy, Brahe) • Milky Way is made up of stars • Odd shape of Saturn • Breakthrough for Copernican model • Inquisition, house arrest, forgery • ‘Eppur si muove’ and still, it moves
Isaac Newton (1643-1727) • Graduated at age 23. • Did fundamental work at age 23 (published later), see paradigm shift • Most well-known for his work on calculus and gravitation/mechanics • Provided quantifiable, causable explanations for the movements of the planets • Could explain planetary motions by three simple laws of motion • All motions!!! Apple, remember? • Philosophiae Naturalis Principia Mathematica (1687)