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UT AST 350L - Lecture 2- Antiquity I

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The History and Philosophyof Astronomy (Lecture 2: Antiquity I)Instructor: Volker BrommTA: Amanda BauerThe University of Texas at AustinAstronomy 350L (Spring 2005)Astronomy and Cosmology in Antiquity:Two Threads of Thought • Mainstream (orthodoxy) Antiquity I (Jan. 20)- Plato, Eudoxus, Aristotle, Hipparchus, Ptolemy- Two-sphere-universe- Earth-centered (geocentric)- Planetary motion: in circles, deferent-epicycle• Dissent (heterodoxy) Antiquity II (Jan. 25)- Pythagoras, Democritus, Epicurus, Stoics, Aristarchus- Democritus (atomism) and Aristarchus (Sun-centered) - close to modern world view - but forgotten (suppressed) for 1,400 yearsAncient Greece: The Birth of Science6thcent. BC: Use geometry to address celestial motionsObserving the Sky: The Basic Facts• Earth is a Sphere• Daily motion of celestial sphere (stars)• Stars don’t change their relative positions• Annual motion of Sun with respect to stars• Moon’ s motion w.r.t. to fixed stars• Planets motion w.r.t. to fixed stars weird(all with the naked eye!)Spherical shape of the Earth• Ships at sea• Lunar eclipse: earth’s shadow circular• Traveler’s Tales (e.g., recorded by Herodotus)Size of the Spherical Earth• Use geometry and common senseEratosthenes (3rdcent. BC, Alexandria)-7o= 800 km-360o = 40,000 kmDaily motion of the starsNo change in relative positions fixed starsDaily motion of the stars: Looks different in different directions!Due NorthDue EastDue South- Circumpolar starsQ: How to explain daily stellar motion???Celestial sphere(contains fixed stars)Celestial equatorLocal Horizon(every observerhas own one!)A: The Ancient Two-sphere-universe!Quick reminder: Why does concept of celestial sphere work (from our present-day perspective)?Ancient Two-sphere-universe: - Plato’s philosophy demands that universe is spherical!• Plato (4thcent. BC)• Timaeus: Theory of thecosmos (and its creation)Plato’s philosophy demands that universe is spherical!Q: How so?Divine craftsman (Demiurge)Realm of IdeasRealm of Experience(William Blake, 1757-1827)Plato’s philosophy demands that all natural motion isuniform along circles!Divine craftsman (Demiurge)Realm of IdeasRealm of Experience(William Blake, 1757-1827)Uniform, circular motionAncient Two-sphere-universe: - Next Q: What is rotating? Earth or Sphere of Fixed Stars???Hypothesis: The Earth?- actually proposed by Heracleides of Pontus (4thcent. BC)- that obviously can explain observations (and we nowknow that it is true)But: Why was this (correct) hypothesis rejectedand rediscovered only ~2,000 years later?Ancient Two-sphere-universe: Q: Why was rotating-Earth hypothesis rejected?A: - Theory of motion (terrestrial physics Aristotle)- Common-sense (naïve expectation)Greeks argued: Stone would be left behind if Earthrotated! (Think about why this argument is wrong!)Stone/arrowObserverQ: How do we know that Earth rotates?A: Foucault’s pendulum (1851)!Ancient Two-sphere-universe: - Q: What is rotating? Earth or Sphere of Fixed Stars???Greek’s (incorrect) Answer: The Celestial Sphere!Q: How could they have gotten this so wrong?1. Conforms to naïve experience2. Elegantly explains many observations3. Backed up by Aristotle greatestauthority for 2,000 years (`The Philosopher’)Two-sphere-universe + stationary Earth:Nicely accommodates annual solar motion!Sun moves w.r.t. fixed stars along ecliptic!Two-sphere-universe + stationary Earth:Nicely accommodates annual solar motion!Sun moves along eclipticonce a year!Two-sphere-universe + stationary Earth:Nicely accommodates annual solar motion!Plato’s Grand Challenge: How do planetary motions fit in?• Retrograde motion of planets, oppositedirection to daily motion (E-W) of celestial spherePlato’s Grand Challenge: How do planetary motions fit in?• First taken up by his pupil Eudoxusfounder of Greek mathematical astronomy •Theory of homocentric spheres (all spheres have sameCenter)Plato’s Grand Challenge: How do planetary motions fit in?• First taken up by his pupil Eudoxusfounder of Greek mathematical astronomy •Theory of homocentric spheres (all spheres have sameCenter) • A many-sphere universe!• How to establish the order of spheres?- Order of planets (Earth, Sun,Moon, Mercury, Venus, Mars…)- What object is in the center?Ancient Two-sphere-universe: - Part of Aristotle’s all-embracing, coherent worldview!• Aristotle (4thcent. BC)• Plato’s disciple, Alexander’steacher• `The Philosopher’- Supreme intellectual authority- Unchallenged till RenaissanceThe Aristotelian Universe: • Earth is in center!• Planets, including Sun, movearound earth, affixed to crystalspheres• The Universe is finite, has edge• Two distinct regions of the cosmos:(1) The Heavens (supralunar)- perfect, no change, circular motions(2) Terrestrial (sublunar)- change (turmoil), non-circular motionsReminder: How do we know that Earth moves?• Not observed (too small) until 1838 (Bessel)!• from our modern (heliocentric) perspectiveThe Aristotelian Universe:The Aristotelian Universe: • A coherent framework of all of nature• Astronomical concepts tied up withterrestrial physics (theory of motion)• Theory of gravity depends on Earth being in center of the universe!• Finite universe, bounded by spherical edge• There cannot exist a vacuum (plenum theory)• Cosmos is eternal, guaranteed by spherical motionThe Aristotelian universe: • Qualitative, but it is not precise as far asplanetary motion is concerned• Greeks before Alexander the Great (356-323 BC)did not care much about observations• Fundamental change in history of astronomyin the wake of Alexander’s conquest (Hellenistic Age)The Hellenistic Age: Alexander’s conquestThe Hellenistic Age: Alexander’s conquest • Greece (before Alexander):- Science and philosophy- Disregard for empirical facts (observations)• Babylon / Egypt:- No Science and philosophy- Wealth of data (observations)Birth of Hellenistic Astronomy:- Quantitative, precision-driven- based in Alexandria (Great library)- Hipparchus, Eratosthenes, PtolemyThe Great Library in AlexandriaHipparchus (2ndcent. BC): Precession of the Equinoxes • slow movement (~26,000 yrs) of CE-ecliptic intersectionThe Ptolemaic System: - Aristotelian, but dominated by mathematical precision!• Ptolemy (2ndcent. AD)• Greatest astronomer ofantiquity• `Almagest’ (150 AD)Ptolemy’s Almagest (Arabic), or Syntaxis:The Ptolemaic System: •


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