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ASTB03 Midterm Study Notes.docx

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Dan Silver

ASTB03 Study Notes Celestial Sphere: - sky appears as shell - stars attached, carried as shell rotates - provides background for observations Suns Motion (ancient) - Sun and Celestial Sphere move from east to west around Earth - Sun not attached to CS - Sun is slower then CS by 1 degree/day - 365 ¼ days for sun to return to same position - Sun appears to revolve in 24hrs…. CS rotates in 23hr 56min Moons Motion - moon moves 13 degrees per day relative to CS (eastward) - 27.3 days for complete cycle - moon phase : 29.5 day cycle (lunar month) Planets - 5 objects visible to naked eye (mercury, venus, mars, Jupiter, Saturn) - usually move steadily eastward relative to CS - retrograde motion Role of Astronomy Timekeeping : Calendars- Egypt—3000 BC, China—14 century BC, Americas--500AD Spiritual/religious events Recurring seasonal phenomena Astronomers= priests Greek Astronomy: Beginning f Scientific approach (600BC-200AD) Geometry - nature described by perfect geometric objects (plato) - straight lines, circles, spheres, uniform motion - Eudoxus (409BC) – planets attached to rotating geocentric spheres – spheres mathematical abstraction Aristotle (384 BC) Spherical Earth: - new stars appear when traveling north or south - curved shadow of earth during lunar eclipses - elephants in Morocco and India Stationary Earth: -objects would be left behind by rotating or revolving Earth (basic Aristotelian physics) - moving earth would produce parallax shift in stars positions Astronomical Measurements Aristarchus (310BC): -relative distances or moon and sun - based on observations of timing moons phase found- sun 19x further then moon (correct 392x) found sun 7x diameter of earth (correct 109x) Eratosthenes (276 BC) - finds absolute size of earth - observations of positions of sun from 2 locations ( Alexandria and Syene) - shadows indicated 7 degrees between cities - circumference is 360/7 x distance between cities - known distance means circumference of earth is 250,000 stade (40,000 kms) Motion in Greek Models Apollonius (220 BC) - introduced eccentric circles, deferents and epicycles - planets paths not circular - epicycle’s needed to explain retrograde motion Hipparchus (190BC) - catalogue of star positions - distance of moon 64 x radius of Earth - modified Apollonius eccentric model Magnitude Scale for Brightness - visible stars ranked into 6 brightness categories: magnitudes - brightest: 1 magnitude , dimmest: 6 magnitude - observed brightness = apparent magnitude (m) - higher apparent magnitude = dimmer.. magnitude can be negative for bright objects - Sun: m= -26.8 , Moon: m= -12.5 , light bulb: m= -19, Venus: m= -4.4 Ptolemy Almagest (140 AD) - star catalogue - circumference of earth = 180,000 stade Geocentric Model of Universe - spherical, stationary earth at center - rotating celestial sphere - deferent + epicycle orbits - survived for 1500 years Transition to Renaissance - Almagest passes to Roman, Byzantine, Islamic empires - Instruments improved by Arabic astronomers Models and Theories: Models- describe specific aspects of nature - can be used to predict behavior Theories: - explain and unify phenomena with basic set of ideas - describes many aspects of nature - make testable predictions Geocentric Model: -predicted motion of planets (within limits of observation) useful for timekeeping/navigation The Classical Solar System Copernicus ( 1473) - equant and doubling of moon – disturbing - revived Aristarchus heliocentric model De revolutions Orbium Coelestium (1543) - sun at center , all planets orbit the sun, moon orbits earth - stars are on celestial sphere, at rest - earth rotates on axis ( causes apparent motions) - Mercury, Venus, Earth, Mars, Jupiter , Saturn Retrograde Motion - result of relative motions of earth and plaents - predicts periodic parallax shift of stars - banned 1616 Tycho Brahe (1546) - Alfonsine and Prutenic tables disagreed on planets motions - New star (nova) 1572 , lack of motion meant it was on CS - More accurate predictions require better observations- prolific instrument maker – repetition of measurements – calibration - Accuracy of 1 arcminute (1’) – 1 degree = 60 arcminutes , 1 arcminute= 60arcseconds - 1577 – comet path crosses other planets orbits , therefore solid crystal spheres do not exist - hybrid model- earth at center, moon and su
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