Islamic Innovatorsand Copernicus
Problems with Ptolemy Equant violates Plato’s rule (uniform,
circular motion) No absolute distances, i.e., not a
physical picture of the cosmos Predicted change in lunar apparent
size not observed (if mechanisms are“real”)
Ad hoc links of all models to Sun
Task of lecture Arabic and Latin medieval critiques of
Ptolemaic astronomyGeocentrism vs heliocentrism not a simple
choice from observationsDoes physics rule astronomy?
How radical or novel was Copernicus? Problems with the Copernican
heliocentric cosmos
Moving Earth stellar parallax?
Earth
Sun
GeocentricHeliocentric
Earth
Sun
Expect no parallaxExpect parallax, notobserved until 1837
Moving Earth Venus parallax?
Earth VenusParallax angle
GeocentricPar (Ven) > Par (Sun)
S
VE
HeliocentricPar (Ven) > or < Par (Sun)Not observed until 1930s
S
VE
Moving Earth birds left behind?
re = 6400 km
Equator
Vequat = 1700 km/hr
Arabic astronomy, 850 - 1300 Islam and the need for astronomy
Prayer TimesDirection of MeccaQibbla for mosquesTypical astrological/medical concerns
Court translations of Ptolemy, Aristotle“Filosofa” (“foreign” philosophy)
Large observatories and “professionals”
Arabic critiques of Ptolemy Improve predictive accuracy
– Correct Ptolemaic parameters with obs.– Add “trepidation” to Ptolemy’s precession– Produce new tables to find positions (zijs)
Fix the physics (hay’a=real, physicalcosmology in Platonic-Aristotelian tradition)
– Ibn al-Haytham (d. 1040) defines hay’a as a problem tobe solved
– al-Bitruji (1200) replaces epicycles & equants withhomocentric spheres (model does not work)
– al-Tusi (1250): replace equant with Tusi-Couple– al-Shatir (1350): replace equant with double epicycles
Tusi Couple
a
2a
Uniform circular motion makes linear harmonic motion!
Medieval Latin astronomy Scholastics synthesize Aristotle and Christian
theology (Thomas Aquinas) Angels move celestial spheres Astrology accepted for everything but human will
Medieval universities Trivium (grammar, logic, rhetoric) Quadrivium (astronomy, music, geometry,
arithmetic) Attempted reforms in Latin West (Arabic
influence)? Peurbach/Angelus & Tusi (1460)? Regiomontanus & homocentrism (1460)
Copernicus, the isolated canon Born in Torin, merchant’s son Studied law, medicine in Cracow and Italy,
acquired best Latin astronomy books in print Time of printing, Columbus, fall of Constaninople
To Poland as Fromberg canon in 1503 The closet astronomer
Observations begin 1503 Commentariolus, manuscript written ca. 1508 De revolutionibus orbium coelestium [On the
revolutions of the heavenly spheres], 1543(Copernicus’s only published book)
A “Copernican Revolution”? The traditional view
– Moving earth from Greeks (Philolaos, Heraclides)– Explained solar links, retrograde motion “naturally”– Osiander’s “instrumentalist” preface– Better predictive accuracy than Ptolemy
New view: A “conservative revolutionary”– Employed Tusi’s mathematics to eliminate equant– Sought hay’a goal of restoring Plato’s uniform, circular
motion– Predicted Ptolemy’s positions, not improved accuracy– But did add novel aesthetic considerations (‘common
measure’)– Created problems for Aristotelian physics?– Refuted by lack of observed stellar parallax?
Coeprnican advantages
>>Explains retrograde asEarth “passing” outerplanet
>>Explains links of innerplanets to Sun as theirorbits around Sun
Steps to heliocentrism: 1
Ptolemaic SuperiorPlanet
r = epicycle radiusR = deferent radius
Mars
Earth
Sunr
R
Steps to heliocentrism: 2Mars
Earth
Sun
Complete theparallelogram
Steps to heliocentrism: 3
Add circle aroundEarth, circle aroundSun = “Tycho’smodel” for SuperiorPlanet
Mars
Earth
Sun
Steps to heliocentrism: 4
Let Earth orbit Sunrather than Sun orbitEarth, i.e., shift from“Tychonic” toCopernican model
Sun
Mars
Earth
Steps to heliocentrism: 5
Sun
Mars
Earth
Copernican
r = Earth’s distance =AU = “commonmeasure” of solarsystemR = Mars’ distance
From Ptolemy, R=60,r=39.5, thus R/r=1.52or Mars is 1.52 AUfrom Sun!
r
R
Aesthetics of new cosmology “… in this arrangement we discover a marvelous
commensurability [common measure] and an establishedharmonious linkage between the motion of the spheres and theirsizes, such as can be found in no other way.” De revolutionibus,Book 1
AU is “common measure” of planetarydistances from Sun
Planetary periods proportional to theirdistances from Sun
Mer .24 yrs, Ven .62, Ear 1, Mars 1.9, Jup 11.9, Sat 29.4
Heliostatic not heliocentric!Earth
Center of Earth’sOrbit
Sun •Sun fixed•Center of Earth’sorbit moves•Requires 3 circles
Copernican cosmology Eliminates equant with Tusi Couples (Arabic
influence) Provides “common measure” for distances and
“harmonious linkage” of period-distance– Aesthetic contribution
Heliostatic rather than heliocentric Retains small epicycles for each planet (not for
retrograde motion) Same number of circles as Ptolemy Same format as Almagest “Saves” Ptolemy’s predictions, not the heavens Creates crisis for Aristotle’s physics
– What is the status of Copernican model (physical realityor astronomical hypothesis)?