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I. Early History of Astronomy A. Ancient Greeks 1. Used philosophical arguments to explain natural phenomena 2. Also used some observational data 3. Most ancient Greeks held a geocentric (Earthcentered) view of the universe 4. Air, Earth, Fire, Water 5. Aristotle-spherical earth Geocentric animation I. Early History of Astronomy a. "Earth-centered" view 1. Earth was a motionless sphere at the center of the universe 2. Stars were on the celestial sphere a. Transparent, hollow sphere b. Celestial sphere turns daily around Earth b. Seven heavenly bodies 1. Changed position in sky 2. The seven wanderers included the a. Moon b. Mercury c. Venus d. Sun e. Mars f. Jupiter g. Saturn Aristotle I. Early History of Astronomy 6. Anaxagoras (500-450 BC)moonlight physics 7. Aristarchus (312–230 B.C.) was the first Greek to profess a Sun-centered, or heliocentric, universe He also calculated relative sizes and distances of the sun,earth, moon I. Early History of Astronomy 8. Eratosthenes (276-194 BC): a. 1st to accurately measure the earth’s circumference using a gnomon and the sun. (250,000 stadia) b. Error of only ~500 miles I. Early History of Astronomy 9. Hipparchus (190120BC): a. logged ~ 850 star locations and star brightness (6 different magnitudes) b. Great mathematician (trig) These are the apparent magnitudes of the bright objects seen in the sky. I. Early History of Astronomy 10. Planets exhibit an apparent westward drift a. Called retrograde motion b. Occurs as Earth, with its faster orbital speed, overtakes another planet animation Retrograde movement I. Early History of Astronomy 11. Ptolemaic system (Claudius Ptolemy) a. A.D. 141, The Almagest b. Geocentric model c. To explain retrograde motion, Ptolemy used two motions for the planets 1. Large orbital circles, called deferents, and 2. Small circles, called epicycles Animation of epicycles and deferents I. Early History of Astronomy …Ptolemy’s Epicycles and Deferents… I. Early History of Astronomy …Ptolemy’s retrograde motion… I. Early History of Astronomy …a model built to explain Retrograde Motion in both a Geocentric and Heliocentric design… RetroLink I. Early History of Astronomy B. Birth of modern astronomy 1. 1500s and 1600s 2. Five noted scientists a. Nicolaus Copernicus (1473–1543) 1. Concluded Earth was just a planet 2. Constructed a model of the solar system that put the Sun at the center, but he used circular orbits for the planets 3. Heliocentric Solar System 4. Ushered out old astronomy 5. De Revolutionibus I. Early History of Astronomy b. Tycho Brahe (1546– 1601) 1. Precise observer 2. Tried to find stellar parallax: the apparent shift in a star's position due to the revolution of Earth 3. Did not believe in the Copernican system because he was unable to observe stellar parallax Animation of different systems I. Early History of Astronomy Tycho Brahe (1546–1601) 4. Great instrument maker Cassiopeia…1572 Tycho’s Supernova Tycho Crater on the Moon The Quadrant I. Early History of Astronomy c. Johannes Kepler (1571–1630) 1. Ushered in new astronomy 2. Planets revolve around the Sun 3. Student of Tycho Brahe 4. Strong mathematician I. Early History of Astronomy 5. Three laws of planetary motion a. Orbits of the planets are elliptical b. Planets revolve around the Sun at varying speed (Faster at perihelion…..slower at aphelion) c. There is a proportional relation between a planet's orbital period and its distance to the Sun (measured in astronomical units (AUs)—one AU averages about 150 million kilometers, or 93 million miles) P2=a3 I. Early History of Astronomy …What is an Ellipse?... I. History of Early Astronomy I. Early History of Astronomy ..the properties of an ellipse… I. Early History of Astronomy Kepler’s 3 Laws: #1: All orbits are elliptical……………..e is eccentricity As “e” approaches 0, the orbit becomes more circular! Icarus’ orbit I. Early History of Astronomy Kepler’s 3 Laws: #2: Equal Area Law animation This implies a planet moves faster at perihelion and slower at aphelion! I. Early History of Astronomy Kepler’s 3 Laws: #3: P2 (years) = a3 (AU) For Earth: Period is 1 year a is 1 AU Therefore (1)2=(1)3 For Jupiter: Period is 11.86 years a is 5.20 AU Therefore: (11.86)2 = (5.20)3 140.6 = 140.6 I. Early History of Astronomy …Kepler’s 3 Laws of Planetary Motion… I. Early History of Astronomy d. Galileo Galilei (1564–1642) 1. Supported Copernican theory 2. Used experimental data 3. Used an astronomical telescope in 1609 4 Galilean Satellites: Io, Europa, Ganymede, Callisto I. Early History of Astronomy 4. Galileo's discoveries using the telescope a. Four large moons of Jupiter b. Planets appeared as disks c. Phases of Venus d. Features on the Moon e. Sunspots and sun’s rotation I. Early History of Astronomy s I. Early History of Astronomy 5. Tried and convicted by the Inquisition…died under house arrest 6. In 1992, 350 years later, Pope John Paul II officially declared Galileo innocent. 7. Galileo died, blind and under house arrest, on January 8, 1642. On Christmas Day of that same year, born was………… I. Early History of Astronomy e. Sir Isaac Newton (1643–1727) 1. Law of universal gravitation 2. Proved that the force of gravity combined with the tendency of a planet to remain in straightline motion results in the elliptical orbits discovered by Kepler 3. 3 laws of motion (inertia…1st law) 4. Mass vs Weight 5. Principia Orbital animation Halley’s Comet I. Early History of Astronomy …What is Gravity?... I. Early History of Astronomy …What is Gravity?... I. Early History of Astronomy …What is gravity?... I. Early History of Astronomy …What is Gravity?... Zero G drink I. Early History of Astronomy …Orbital Movement… Link I. Early History of Astronomy …A brief timeline review………