The History and Philosophyof Astronomy (Lecture 15: Enlightenment: Kant/Herschel)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)Timeline: The Enlightenment PeriodQuestion: What is Enlightenment???Enlightenment Philosophy • Immanuel Kant (1724-1804)• Critique of Pure Reason (1781):- Big Q: What can we know?• Theory of the Heavens (1755):- “Island Universes” (galaxies)Question: What is Enlightenment??? “Enlightenment is man’s emergence from his self-imposedimmaturity. Immaturity is the inability to use one’s understanding without guidance from another. This immaturity is self-imposed when its cause lies not in the Lack of understanding, but in lack of resolve and courage to use it without guidance from another. Sapere aude! [dare to know] Have courage to use your own understanding!— that is the motto of the Enlightenment.”(Kant, 1784)Enlightenment Philosophy • Jean-Jacques Rousseau (1712-1778)• The Social Contract (1762):- “ Man is born free but everywhereis in chains”- Purpose of government: to ensurelife, happiness, general welfare- bad government: social contract isbroken! Change of governmentlegitimate!Political Upheaval: American Revolution “Life, liberty, and the pursuit of happiness…”Political Upheaval: French Revolution (1789) “Liberte, egalite, fraternite!”Economical Upheaval: Industrial Revolution • James Watt:- Steam Engine (1774)• beginnings of mass-production (Taylorism)• Adam Smith (1776):- Wealth of Nations(“Bible of Capitalism”)- Theory of the “invisiblehand”The Birth of Chemistry • Antoine Lavoisier (1743-94), discoverer of oxygenBig Q: Architecture of Stellar System? • Q: How are stars arranged, and are they really fixed?Newton’s A: Static, infinite Universe! • Stars are fixed; i.e.: they don’t move!But what about “Milky Way”band of stars? • Q: How was Milky Way, and other galaxies, discovered?But what about “nebulous patches”of stars? • Many “islands of stars” !Newton: A Static Universe • Q: Is such a Universe possible (is it stable)?Newton: A Static Universe • Can stellar system be stable under gravity?• A: No! Analogue: Balance infinitely many needlesexactly on their tips! An unstable situation!Newton: A Static Universe • A: No, stars would begin to move around randomly!Hunting for Stellar Motions • Slow shift in angular positions proper motions• unsuccessful searchinitiallyproper motions areexceedingly smalldue to large distanceto the stars!Hunting for Stellar Motions • proper motion 1/ddSerendipity: Bradley’s Discovery of Aberration • Apparent stellar positions are shifted because ofEarth’s motion around Sun!Bradley’s Proof for Earth’s Motion • James Bradley (AstronomerRoyal, 1693-1762)• 1728: `Aberration of Light’• Incontrovertible prooffor Earth’s revolutionaround Sun (Copernicus was right!)Serendipity: Bradley’s Discovery of Aberration • Apparent stellar positions are shifted because ofEarth’s motion around Sun!Serendipity: Bradley’s Discovery of Aberration • Apparent stellar positions are shifted because ofEarth’s motion around Sun!Q: How are Stars distributed in infinite Space? • Stars are in motion, but what is architecture of stellar system?ORExplaining the Milky Way! • Thomas Wright ofDurham (1711-86)• An Original Theory ofthe Universe (1750)• Concept of Milky Wayas a `Galaxy’(distinct system of countless stars)Wright Explains the Milky Way! Divine CenterOuter DarknessWright Explains the Milky Way! Very large RR• a disc-like appearance (like observed band of MW)!Enlightenment Philosophy • Immanuel Kant (1724-1804)• Critique of Pure Reason (1781):- Big Q: What can we know?• Theory of the Heavens (1755):- “Island Universes” (galaxies)Kant Postulates Disc-like Milky Way! • basically correct picture for our Milky Way!• direct proof:COBE, 1993(infrared picture)Kant’s Cosmogony:Formation of disc-like systems! • Kant-Laplace hypothesis!Kant’s Hierarchy of Worlds Island Universes(`galaxies’)• Systems of systems • a fractal universe!The Fractal Geometry of Nature • Benoit Mandelbrot • Fractal: self-similarityThe Fractal Geometry of NatureEnlightenment Philosophy • Pierre-Simon de Laplace(1749-1827)• Exposition de System du Monde(1796)• Laplace Demon:- a being endowed with unlimitedcapability to compute- would be able to predict everythingin past, present, and future- a completely deterministic universe!Enlightenment Astronomy • William Herschel (1738-1822)• Discovery of Uranus (1781):- First planet not known in Antiquity• On the Construction of the Heavens (1785):- star counts  Map of the Milky Way• Greatest telescope-builder of the ageHerschel: From Musician to AstronomerOrganist in Bath (1766-82)At age 35, Herschel turns to astronomy!• Composer, conductor,oboist, organistHerschel: Greatest Telescope-builder of the AgeCaroline Herschel (1750-1848)40-foot reflectorHerschel: Discovery of Uranus (1781)Voyager 2 (1986)7-foot reflector• First planet not known to the Ancients already!Herschel: Mapping the Milky Way• Using star counts to estimate distanceto the edge of Milky Way Shape of Milky Way Galaxy~10,000 lightyearsHerschel: Discovery of Infrared Radiation (1800)• Use prism to dispersesunlight into spectrum(blue-green-yellow-red)• Place thermometer at“empty” place beyond red• Detect heat there there must exist energy(radiation) beyond red= InfraredGreat Task Ahead: What arethe Stars? • still unsolved: Need better physics and chemistry!Enlightenment Astronomy (Kant/Herschel)• First promising explanations for stellar architecture: - Milky Way- an infinite system of `island universes’ (galaxies)- maybe universe is a fractal (infinite regression of worlds)?- no incontrovertible proof yet: need stellar distances!•