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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


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


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


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"


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


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

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


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


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



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