385L37
Ptolemaic Contributions • consolidation of earlier models/theories • extension and refinement of Hipparchan trigonometrics • refinement of armillary sphere • data-driven expansion of model • epi-epicycles • mobile eccentricity • equant orbit
Armillary Sphere • Eratosthenes credited with invention; developments by Hipparchus and Ptolemy • geocentric model • rings represent equator, ecliptic, celestial latitudes and longitudes • plots solar and stellar positions and angles • observational instrumentation
Retrogression • exact speed of retrogression varies over time • length of retrograde loops varies : shorter (fast) vs. longer (slow)
Epicycle & Eccentric • epicycle predicts only uniform retrograde loops • eccentric predicts regular changes in angular velocity • eccentric + epicycle predicts irregular seasonal lengths • neither predicts both position and speed of retrogression
Ptolemaic Modifications • re-arrangement of existing models: epi-epicycles • new combinations of models : mobile eccentrics • equant orbits
epi-epicycle (Ptolemy)
Mobile Eccentric (Ptolemy) planet (P) on epicycle orbiting point (C) on deferent point (C) orbiting point (G) on epicycle point (G) orbiting eccentric point (F), where E = Earth
equant (Ptolemy)
Uniform, Eccentric, Equant Orbits
Eccentric, Equant, Retrogression
BODY EUDOXOS KALLIPPOS ARISTOTLE PTOLEMY Moon 3 5 Sun 6 Planets 20 23 44 68+ Stars 1 TOTAL 27 34 56 80+
Astronomical Premises 1 sphericity of universe and all celestial objects 2 circularity of all celestial motion 3 perpetual nature of all celestial motion 4 uniformity of all celestial motion 5 uniformity of celestial position/place 6 geocentricity 7 stationary Earth 8 rotation of celestial sphere
• Eudoxan spheres consistent with all premises • epicyclic model violates uniform place (5) premise • eccentric and equant models violate circular (2) and geocentric (6) premises
Ptolemaic Universe http://www.youtube.com/watch?v=EpSy0Lkm3zM
Ptolemaic Contributions • consolidation of previous models • rigorous mathematization, esp. measurement of angular velocities • enhanced predictability of celestial events (star tables, ephemerides, navigational charts) • reduction (NOT elimination) of some contrary observational data, creation of new contradictories • triumph as mathematical model of universe, failure as astronomical model • skewing of physical astronomy (Aristotle) and mathematical/technical astronomy (Ptolemy)
Mathêmatikê Syntaxis [Mathematical Compilation] (ca. 150 CE) Megistê Syntaxis [Greatest Compilation] Arabic Al-magest [The Greatest {Book}] (9th century) Latin Almagest (12th century) Copernicus (1473-1543)