Cosmology in Ancient Greece: Aristotle and Greek Astronomy

ASTRONOMY 9: HISTORYOF COSMOLOGY Handout #10 Cosmology in Ancient Greece: Aristotle and Greek Astronomy I. Aristotle (384 322 DC) Student of Plato's Academy Founded Lyceum Worked in alnlost every field of 1inon.n science and philosophy! Profound influence on develop~nentof \Vestern science Ever?- effect must have a cause Important innovation: unil-erse can be described by natural laws inferred by rational thought Aristotle's physics: mostl?- quite wrong, but strong co~n~no~l-sense appeal - Founded science of mechanics (physics of motion) - Del-eloped the idea of "force", impetus theory of notion (wrong, but widely held even today!) - ,Thought force was required to keep Earthly objects in motion (didn't fully understand %ncrl%a) Aristotle applies his physics to the cosmos - Basic Earthly elements: air, earth, fire, r a t e r - 'Terrestrial and celestial physics are very different! - ,Two types of "natural" motion A) Earth: linear, straight-line, finite notion (air and fire go up. water and earth go domrn) D) Heavens: perfect. eternal, circular motion - Bodies are impelled to move to their "natural" location - Can onl?- have one center 3 only one world - Circular motion nlust ha>-e a corresponding basic elenlent 3 5th element (quintessence): ether (heavenly, unchanging) - Cosmic division: A) Sub-lunar?- sphere (inside moon's orbit): earthly. changing. becoming, imperfect, made of 4 ele~nents(Ionian) D) Celestial realm (moon and beyond): immutable, perfect, made of ether (Eleatic) - Objects don't get "left behind': 3 Earth is fixed at the cosmic center - Earth known to be spherical on obscrual%onal grounds: A) Circular shadow during lunar eclipse D) Different stars become visible mrl~entravel north/south - Heavens are rotating 3 Universe nlust be finite! - Space has an unapproacl~able"edge" - Universe is eternal. untreated - God rules from outside (primary cause of all motion) - Appropriates Eudoxus' sphere nod el to explain planetary notion - Better observations require more spheres 3 55 total! (Still no esplanation for varying brightnesses) - Aristotle thinks spheres are physically real - No "\Toid" (vacuum), unlilie the Atomists 11. Heraclides of P o ~ ~ t u(c. s 388 31U IIC) Proposed rotating Earth to explain daily rotation of heavens Proposed >Iercur?- and Venus orbit Sun, not Earth Ideas rejected by Aristotelians 111. Aristarchus of Sa~nos(approx. 310 230 BC) "The Ancient Copernicus" hfeasuri~igthe size of the cosmos... Estimated size of Moon to be = 113 size of Earth (from lunar eclipse shadow-) Measured angle betvieen Sun and Moon when hloon is half-full (called "first quarter") Concluded Sun is 18 2Ux further away than Moon But apparent (angular) sizes of Sun and 14oon are the same 3 Sun is at least 18 times as big as 140011, or 6 times as big as Earth! Used inaccurate data (Moon diameter 2', actuall?. 0.5'), and method doesn't w-ork vie11 in practice: Sun is actually mucli bigger still! Heliocentric model: Sun is a t the center! Correct but rejected b?- Aristotelians, forgotten for 1500 years ... Science is not a linear progression of ideas! IV. Eratosthelies of Cyrene (276 194 DC) Clever measurement of the circumference of the earth Distance between Alexandria and Syene (now Aswan, Egypt) measured to be 5000 "stadia" (Greeli unit of length) Compare noon shadon-s a t summer solstice, find 7.2' difference Assume Sun is far am-ay so that rays are parallel Circumference is then C' = (g) x 5000 stadia = 50 x 5titiU stadia = 250, ti00 stadia, Length of stadium is uncertain; result vias very accurate if 1 stadium = 157.2 meters: C' = (250. 000 stadia) x 1 = (250, UOU stadia) x 157.2 m - ( 1 stadia) (-1 ' i "'lim 1.. Appollo~iiusof Perga (approx. 262 190 BC) Back to the geocentric cos~llos A nevi answer t o Plato's challenge to %a\-e the phenomena" Different from Eudoxus' "onion" cosmos of spheres, more like a Ferris wheel Not totally geocentric: introduced "eccentric circle" - Eartli renlains nlotionless a t cosnlic center - Planets still move in circles at uniform speed - But center of circles is ~ i o w displaced from Earth! - Apparent motion as viewed from Eartli is of variable speed Also in\-ented epicycle - Wheels upon wheels - Allows (complicated) explanation of retrograde nlotion "Theorist": did not tr?- to apply ideas to observational data VI. Hipparchus of Rhodes (190 120 DC) Great Greeli astro~iomer Synthesized Babylonian and Greek data with new Greek geometrical models from Appollonius - hfodel for Sun's motion (eccentric circle) - hfodel for hloon's motion (eccentric epicycle) Also made accurate catalog of 850 star positions Disco\-ered precession of equinoxes +