THE PHYSICS/COMOLOGY CONNECTION: I
From Ptolemy to Kepler
Part I: The Platonic/Aristotelian Legacy
Eudoxus: Nested Spheres
Apollonius and Hipparchus: Epicycles (Sun and Moon)
Ptolemy: Epicycles everywhere
Part II: From Geocentric to Heliocentric Cosmos
Copernicus: A conservative “revolution”
Tycho Brahe: Measuring the heavens
Kepler: The first physical laws
PLATO’S LEGACY
• Learn about cosmos from reason: Cosmic Geometer
• Motions in heavens are circular and uniform
• Problem: How to describe irregular planetary motions
with simple circular motions ?
 SAVING THE PHENOMENA
sun
earth
planet
How to explain retrograde motion with circles
in an earth-centered cosmos?
stars
A “solution”: Eudoxus and his nested spheres (c. 408-356 BCE)
• Cosmos as an onion of concentric spheres:
combined rotation reproduces retrograde motion
• Problems: how is motion transferred from sphere to sphere?
how is variable brightness explained?
• Aristotle: tries to improve, using 56 spheres; cosmos as machine
no good for brightness issue
Some transitional (and forgotten) ideas:
• Heraclides (ca. 388-310 BCE)
Earth rotates (a no-go for Aristotelians)
Mercury and Venus orbit the Sun not Earth
(orbits “close” to Sun, shorter than a year)
• Aristarchus of Samos (ca. 310 BCE)
Sun is much larger than the Earth
 SUN IS THE CENTER!!
Parallax
But no parallax (Bessel 1838) + Aristotelian critique: Earth not ether
EPICYCLES TO SAVE THE PHENOMENA
• Apollonius of Perga (265-190 BCE): the idea
• Hipparchus (fl. 150-125 BCE): Greatest astronomer
applied epicycles to the Sun and Moon - variable brightness
(Also invented trigonometry)
• Ptolemy (fl. 127-141 CE, Alexandria)
-Almagest dominated astronomy until 16th century
-description of ALL heavenly motions
-Order is manifestation of superior reason
• Ptolemy’s violation of Platonic rule:
center of epicycle moves uniformly about equant
not about center of deferent
- agreement with observations is important!
• Arabs: Kept Ptolemy’s ideas alive and made improvements
Ptolemy’s
equant
• Middle Ages: Universe remains aristotelian
earth-centered, finite, two realms
• “Obstacles” to science:
Two realms --> two physics
Uniform motion w/ perfect circles
Inability to understand inertia
Geocentrism
Divorcement of math and science
No empirical validation
• Obstacles removed by Copernicus, Kepler, Galileo
• Nicolau Copernicus (1473-1543)
-A reluctant hero, a “conservative revolutionary’’
-Ptolemy’s model violated Platonic rule: no equant!
going back
-Put Sun in center: a better proportion of orbital periods
aesthetics (Me: 3mo.; V:9mo.; M: 2y; J: 12y; S: 30 y)
• 1510: “Commentariolus”: Sun is center of cosmos
Earth spins around its axis
Earth goes around the Sun
(still…lots of epicycles!)
Moon goes around Earth
All orbits are circular
• 1543: On the Revolutions of Heavenly Spheres
Osiander’s preface...
KEPLER (1571-1630)
• First mathematical laws describing natural phenomena:
3 laws of planetary motion
• A Pythagorean: Numbers  Geometry  Beauty!
• Search for PHYSICAL CAUSES of planetary motion
• If theory doesn’t fit data change theory: very modern
• Kepler needs data: 1600 goes to Prague to work w/ Tycho Brahe
• Brahe: greatest astronomer -- precise measurements crucial
1572: Supernova; 1577: Great Comet
 CHANGE IS POSSIBLE IN THE HEAVENS!!
• 1609: Astronomia Nova (genius and sweat)
2 laws: ellipses and equal areas
Sun is -mathematical center of cosmos
-physical
-metaphysical
First law
Second law
• 1618: Harmonice Mundi
-Third law: P2 = a3 (P in years and a in A.U.)
1 A.U. = 1.5 X 108 km
• Physics: gravity as magnetism
-1600: William Gilbert: Earth is a magnet
• Invisible force acting at a distance!
From Astronomia Nova: “ Gravity is the mutual tendency between material bodies
Toward unity so that Earth draws a stone much more than a stone draws Earth”