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Unit1: The Physics of Astronomy This Week & Next: Astronomy in Motion Today: Historical Background & Basic Refresher •Kepler’s Laws •Newton’s Laws •Work & Energy General Note: Please ASK if there are terms/math/physics that are new to you! Why is Astronomy Different? Astronomy is the Science of Observation rather than Experiment X What are the Most Basic Observable Quantities? Motion Light What were the first objects routinely observed? When? A Little History... Astronomy is an ancient science •Maya •Aztec •Egyptians •Chinese •Greek II. Early Scientific Astronomy Plato (350 BC): Geocentric Model of SS -planets & Sun all orbit a stationary Earth - orbits are perfect spheres vs. X Aristarchus (270 BC): First Heliocentric Model -planets orbit the Sun -spherical orbits REJECTED! II. Early Scientific Astronomy Ptolemy (100 AD): added epicycles to better fit observed motion of planets Almagest “The Great Treatise” Medieval Manuscript, Melk Abbey (Austria): Geocentric Solar System! (pic fr. 4/17/09) 15th Century! Medieval Manuscript, Melk Abbey (Austria): Geocentric Solar System! (pic fr. 4/17/09) 15th Century! BUT – Predicted locations of planets, by late 1400s PROVED WRONG III. Groundbreakers Copernicus (1473 - 1543): On the Revolution of Heavenly Spheres New Heliocentric Model BUT... -still Spherical -contained epicycles III. Groundbreakers Tycho Brahe (1546-1601): Best set of astronomical positions to date – * Incredible observer – naked-eye observations – Planetary & stellar positions good to <1’ – Duel: who is a better mathematician???? I sez Copernicus not gud yet - no cheezburgers III. Groundbreakers Johannes Kepler (1571-1630) • Recognized need for model to match Tycho’s observations •New laws of Planetary Motion (1609) What are Kepler’s 3 Laws of Planetary Motion? Kepler’s Laws of Planetary Motion K1: Planets orbit the Sun in an ellipse, with the Sun at one focus of the ellipse K2: A line connecting a planet to the Sun sweeps out equal areas in equal time intervals K3 (Harmonic Law): P2 a3 The squares of the sidereal periods is proportional to the orbital semi-major axis cubed The Physics of Motion! Galileo (1564-1642) Isaac Newton (1642-1747) Principia (1687) Newton’s Laws of Motion N1(Law of Inertia): An object at rest will remain at rest and an object in motion will remain in motion in a straight line at constant speed unless acted upon by an outside force N2: The net force on an object is proportional to the object’s mass & acceleration N3: For every action there is an equal and opposite reaction Newton What is the physical explanation for Kepler’s Laws of planetary motion?? Derivation of Universal Law of Gravity What is the Nature of this Force???