Lecture 4: Aristotle's Cosmos Flashcards Preview

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1

Overview of Aristotle's Model

->Earth fixed at center
->Sun, Moon appear and disappear => planets
->nothing past Saturn, no moons

Two realms:
->Sub Lunary Realm, where rectilinear and made of the 4,
->Celestial Realm, where everything is circular motion and Aether

everything is embedded in crystalline spheres (made of Aether) which rotate

2

What is the Sub Lunary realm made of? Where can you find them?

All made of the EWFW, which have natural motions.
-Fire: Just below sphere of moon
-Water/Earth: Centre of Earth
-Air: Between Fire and E/W

3

Compare movement in the two realms

Sub Lunary: Rectilinear. When objects hit the boundary, they "bounce" and continue on inside

Celestial: Perfect circular motion
->perfect because no net change

4

Biggest issue with circular motion?

Retrogade motion: Planets sometimes loop backwards in the sky, which is tough to explain with perfect circles

Was a real challenge for 2000 years:

"By the assumption of what uniform and orderly motions can the apparent motion of the planets be accounted for"

5

Aristotle: Explain the Earth's location in the cosmos

Earth center:
->moon orbits Earth
->if Earth orbits something, moon's sphere would smash our sphere

Imp: they didn't know inertia, gravity, projectile motion
->balls flew by the air rushing around to behind them

No inertia, so how could Earth move?
->I can jump without hitting the wall
->birds can fly without ditched off Earth
->where are the super winds

6

What was Eudoxus' model, and what were the shortcomings?

He didn't leave any writing, but others picked up on it:

Many connected loops; a gyroscope
->different axis give retrogarade
->Sun, Moon = 3 rings, other 5 had 4 ringss + 1 outside
->total 27, but others made 57 for more accuracy
->Plato worried planets could affect other's motions
->added 50+ "UNRAVELING SPHERES"

Unanswered problems:
->Why do stars blink and planets not?
->Why do planets get brighter? (i.e. closer)
->Why 7 planets

7

What was Ptolemy's model, and why was it favoured?

Spheres within spheres rolling.
->"Epicycles", those little circles on circles

HIGHLY PREDICTIVE ->eclipses to the month

8

Why was Geocentric system kept for so long?

Aristotelean methedology very convenient
->makes astronomy very difficult with no apparent gain to go Heliocentric
->need something good to replace Arist before switching

9

Aristarchus?

Measured the size of the cosmos (Sun, moon) in Re