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Ptolemy to Copernicus Wednesday,, January 28 Astronomy of the ancients Many ancient cultures took note of celestial objects and d celestial l ti l phenomena. h T Th They noted t d certain t i patterns tt in the heavens and were ab ble to construct calendars. The Chinese, Egyptians, Brritons, Mayans, and others have left us evidence of th heir interest in astronomy astronomy. Stonehenge can be used as an astronomical calculator. Greek scientist Aristotle showeed that the Earth is spherical. Aristotle supported his statement thhat the Earth is round with observations. The Earth’s shadow on the Moon during d a lunar eclipse is always circular. The only object that always throwss a circular shadow is a sphere The only object whosse shadow is always circular is a sphere. p Aristotle (384 BC – 322 BC) Eratosthenes (ca. (ca 200 BC, B Alexandria, BC Alexandria Egypt) measured the Earth’s circumference: Eratosthenes: 42,000 42 000 km Actual: 40,000 km Greek astronomers deeveloped a geocentric (Earth-centered) moodel for the universe. Basic assumptions of Greeek astronomers: ● ● ● Spherical Earth is stationary, at the center of the universe. universe Earth is corrupt, heaven ns are perfect. Heavenly bodies move with w uniform circular motion. ti Bad assumptions bad conclusions. Hipparchus of Rhoodes (190-120 BC) Important early astronomer: ● catalogue of 1000 stars ● classified stars by brightness ● discovered precession of the equuinoxes ● determined: obliquity of the ecliiptic ● synodic periods of planets ● inclination of Moon's orbit ● place of Sun's “apogee” ● eccentricity t i it off the th “Sun's “S ' orbit” bit” ● estimate of the Moon's distance,, using the diameter of the Earth as a baseline ● He put astronomy on a geometriical basis. Ptolemy used epicy ycles to explain the retrograde motiions of planets. Ptolemy worked in Alexand dria, was active around AD 140. 140 Used results of Hipparchus’ research and measurements to create a model d l off how h the h solar l system worked Wrote an astronomy text, laater called the “Almagest” ((= “the best”). ) Predicted positions of planeets far into the future that were adequately accuratee. Basic structure of geocentric model: Belief B li f in i Ptolemy’s P l ’ geocentric model l lasted d until il the h 16th century. Cosmographia first published 1524 Cosmographia, Geocentric models havve pproblems explaining p g retrograde mottion of planets. Planets usually move west too east relative to stars; during retrograde motion, motion th hey move east to west. west Ptolemy’s explanationn of retrograde motion: The planet (P) moves in a small circle called the epicycle. The center of the epicycle (A) g moves in a large circle called the deferent. Th he combination of small and he large circles produces “l “loop-the-loop” th l ” motion. ti Ptolemy’s modell: did not fit data During the Middle Ages, Ptolemy’ss model had to Ptolemy be fiddled with – more epicycles were added. added The model was needlessly p because it complicated was based on erroneous p assumptions. OCCAM’S RAZOR entia non sunt multiplicanda ppraeter necessitatem entities should not be multiplied beyond necessity William of Occam (c. 1285–1347 ?) Copernicus proposed a heliocentric model for the uniiverse. Mikolaj Kopernik (1473 1543) (1473-1543) Poland Stated that Sun, Sun not Earth, was at the center of the universe. Basic structure of heliocentric h model: Sun is at center. Earth revolves around Sun Sun. Earth rotates around axis. In the heliocentric model of o Copernicus Copernicus, retrograde motion of planets is naturally explained. Retrograde motions occurr naturally if planets further from the Sun mo ove more slowly slowly. Example: p Earth and Ma ars Earth’s orbital radius = 1 A.U. A Earth’s orbital speed = 30 km/sec Mars’ orbital radius = 1.5 Mars 1 5 A.U. AU k Mars’ orbital speed = 24 km/sec As Earth “laps” p Marss,, Mars appears pp to ggo backward as seen byy observer on Earth. ● Earth catches up with Mars M – ● ● a-b Passes it – b-f – Apparent westward motiion S Sees iit move to east agaain i – g Heliocentric model off Copernicus met with considerable (scieentific) resistance. Why? It implies that distaance from Sun to stars is much h greater than h distan di nce from f S Sun to Earth: E h ● ● Stars do not vary much in brightness over the course of a year. Stars do not show a larg ge parallax over the course of a year year. r The parallax to the nearesst stars is about 1 arcsec (˝) R di l aspects off Copernican Radical C i model: d l ● Earth is not at center. ● E th is Earth i moving. i ● Earth is just another plannet. ● Space is big – REALLY Y big. Conservative aspects of o Copernican model: ● Uniform Circular Motio on assumed. ● E i l still Epicycles till required. i d Few closing g questions: 1) Do the inner planets show h retrograde d motions? 2) See picture on the right. i h Is I it i real? l? 3)) In that picture, p , could yyou have Venus instead of Saturn? (tricky) Few closing quesstions continued: 4) See picture on the right Is it real? right. 5)) In that ppicture, could you have Saturn instead of Venus? 6) Could you have Mercury instead i d off Venus?