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UMD ASTR 120 - Class 2 Early models of the “Universe”

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1Class 2Early models of the “Universe” Ancient ideas and preconceptions Development of the modern (Suncentered) view of the Solar System The scientific method at work…I : Greek Astronomy Greeks seemed to be first culture to study theworld in a systematic and logical manner But this was 2000+ year ago, so extremelylimited technology was available! Put yourself in their shoes… what do your ownsenses tell you about the world and the widerUniverse?2 Greeks knew the Earth was a sphere They understood that Lunar eclipses occurwhen the Moon passes through the Earth’sshadow The shadow is always circular… so Earthmust be spherical3 Greeks knew how big the Earth was! Sun reaches different mid-day “heights” inthe sky at different places on the Earth’ssurface Pioneered by Erathosthenes (276-195 BC) Alexandria to Syene = 5000 strades Sun appears to shift by 7 degrees (1/50th circle) Circumference of Earth =5000st x50=250,000 st. We think 1 strade about 1/6 kilometer… so theygot a circumference of about 42,000 km Idea of a spherical Earth was “ok” onaesthetic/philosophical grounds4Ancient greek’s maps of the world5 So that’s the Earth… what about therest of the “Universe” Most ancient thinking was based on theGeocentric model of Aristotle Earth is at the center of the Universe Earth is still (does not rotate!) The planets, Sun, Moon moves in circlesaround Earth The stars are fixed onto a giant sphere thatsurrounds the whole system There is nothing (no space) beyond thestarsAristotle6Geocentric model This idea fails to explain an importantphenomenon, retrograde motion All astronomical objects move across thesky from East to West But, relative to the stars, planets normallymove (slowly) from West to East Some planets known to the Greeks (Mars,Jupiter and Saturn) sometimes reversedcourse and moved (relative to stars) East toWest… this is retrograde motion! Hard to explain within Aristotle’s geocentricmodel7Retrograde motion Ptolemy added a new ingredient toexplain retrograde motion Suggested a system of “epicycles”8Dept. of Physics andAstronomy,Univ. of TennesseLet’s see how this works…Dept. of Physics andAstronomy,Univ. of Tennesse Even with Ptolemy’s epicycles, the predictedpositions of planets differed from realpositions… needed more epicycles9 What are the predictions of Ptolemy’sgeocentric model and how do theycompare with observations? A revolutionary idea from Aristarchus(310BC-230BC)… Maybe the Sun is at the center, and all ofthe planets orbit around the Sun! Retrograde motion can then be explained asdue to the Earth “overtaking” the otherplanets on their orbits This was motivated by Aristarchus’s firstattempts to measure the ratio of thedistances of the Sun and the Moon… hefound that the Sun must be much furtheraway and hence must be huge! If its sobig, why can’t it be at the center?II : Early Heliocentric models10Aristarchus(ca. 310-230 BC)11 Aristarchus’s idea didn’t catch on… itappeared to contradict observations12 Heliocentric model was finally revivedby Copernicus Many believe that he “discovered” the ideaof the heliocentric model by readingAristarchus Irrespective… this was the beginning of theend for the Earth-centered


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