The History and Philosophyof Astronomy (Lecture 14: Newton II)Instructor: Volker BrommTA: Amanda BauerThe University of Texas at AustinAstronomy 350L (Spring 2005)En Route to the Principia • Newton was very reluctant to publish! • Edmond Halley (1656-1742)• member of Royal Society• Halley’s Comet• first astronomer to observeSouthern Sky (from St Helena)• Convinced Newton to publish PrincipiaNewton’s Principia (1687) • Philosophiae NaturalisPrincipia Mathematica(Mathematical Principles of Natural Philosophy)• Challenges Descartes’Principia Philosophiae (1644)- Descartes: qualitative- Newton: quantitative, predictive• The foundational text formodern physics and astronomy!Newton’s Principia: Overall Structure Book 1: Basic Laws Book 2: Demolition of Cartesian System Book 3: System of the World (applications)Principia Book 1: Basic Laws • Scholium: Absolute Space and Time“Absolute space…without relation to anythingexternal, remains always similar and immovable”“Absolute, true, and mathematical time, of itself,…flows equably without relation to anything external ”• Passive stage for all motionPrincipia Book 1: Basic Laws • Newton’s bucketexperimentPrincipia Book 1: Basic Laws • Newton’s 1stLaw of Motion:• Law of inertia (Galileo, Descartes)• State of rest and uniform motion are equivalent(relative to observer)Principia Book 1: Basic Laws • Newton’s 2ndLaw of Motion:• Same force (F) exerted on a larger mass (m)produces a correspondingly smaller acceleration (a)Force = mass x accelerationPrincipia Book 1: Basic Laws • Newton’s 3rdLaw of Motion:Force = Counter-forcePrincipia Book 1: Basic Laws • Newton’s Law of Gravity:• Gravity is universal and follows inverse-square law!Principia Book 2: Demolish Descartes • Newton proves mathematically:- Descartes’ vortex motion cannot be sustained!- it would soon stop because of friction(objects have to move through `plenum’)Principia Book 3: New System of the World • Newton’s laws celestial motions (ellipse, parabola,hyperbola, circle)Principia Book 3: New System of the World • Newton’s laws Kepler’s Three Laws of Planetary Motion(gravity +laws of motion)Kepler 1Kepler 3Kepler 2Principia Book 3: New System of the World • Newton’s laws ocean tides!Principia Book 3: New System of the World • Newton’s laws true (bulged) shape of Earth!EquatorialBulgePrincipia Book 3: New System of the World • physical explanation for Hipparchus’ observation!Principia Book 3: New System of the World • comets move on highly eccentric orbits around Sun!Halley’s Comet • Next return scheduled for 2061!1986: Giotto missionPrincipia Book 3: New System of the World • stoic universe (infinite space, finite matter) is unstable!stars“void”Principia Book 3: New System of the World •Need Epicurean universe (infinite space, infinite matter)!The Infinite Universe: Letters to Bentley (1692-93) • Richard Bentley (1662-1742)• erudite theologian• lectured on how to combatatheism • basic idea: God’s perfectionrequires infinite universe• double-check with Newtonon technical details1688: Glorious Revolution • last Stuart king (James II)wants to re-catholicize England• Parliament asks Protestant rulerof the Netherlands (William of Orange) for help• William invades, James II flees• William (III) and Mary crownednew king and queen• Beginning of constitutional government (`Bill of Rights’)Newton permanently moves to London • 1696-1727• “affairs of state”:- member of Parliament- master of the Royal Mint(oversees large-scalere-coinage program;becomes very rich)• President of the Royal Soc.• stops doing science!Journey’s End: Death in 1727 • buried in Westminster AbbeyNewton: The Legend “Nature and Nature’s Laws lay hid in night;God said, Let Newton be! And all was Light.”(Alexander Pope)“Newton with his prism and silent face,The marble index of a mind for everVoyaging through strange seas of Thought, alone.”(William Wordsworth)Newton: The Legend (William Blake, 1795)• cold, unfeeling rationalist and law-giver, a demigodNewton (part 2)• Isaac Newton: - founder of modern physics and astronomy- early life: led reclusive anti-social life in Cambridge- later life: tended to affairs-of-state in London (master of the Mint)• Principia - foundational text for modern physics and astronomy- laws of motion- universal gravity (inverse-square law)- demolished Cartesian model- explains large number of hitherto unrelated phenomenawithin one unified theoretical framework• The Legend: - “the marble index of the mind for