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The Astronomical Revolution Astronomy in Antiquity Established facts in ancient times: 1. spherical earth - estimate of it’s size 2. lunar phases and eclipses 3. the apparent motion of the sun on the sky 4. sophisticated calendars and time-keeping Outstanding puzzle: the motion of the 5 planets, Mercury, Venus, Mars, Jupiter, and Saturn against the stars We know that the planets move on orbits centred on the sun Heliocentric model - Sun-centred model Early models put the earth at the centre Geocentric Model – Earth-centred model sun, moon and planets move on circular orbits about the earth Recommended reading: The Sleepwalkers, by Arthur Koestler 1 Retrograde Motion outer planets normally proceed from W to E in sky near opposition they reverse direction and start going E to W why? - easy to explain in a heliocentric system apparent path on sky Mars Earth AST121 Lecture 3 AST121 2 In the geocentric model retrograde motion is hard to explain: Many models were developed over nearly a millenium: 1. Pythagoras - circular motion 2. Eudoxos and Aristotle (rotating spheres) 3. Hipparchus - the epicycle 4. Aristarchus - a heliocentric model! Ptolemy’s Almagest - culmination of these efforts planets move on circles about the earth secondary motions about epicycles explains the retrograde motion more than 80 epicycles were built into the whole model! very good agreement with naked eye observations used for 1500 years - passed down to Europeans through Arabic translations AST121 Lecture 3 AST121 3 Mars epicycle Earth deferent AST121 Lecture 3 AST121 4 Copernicus Established the heliocentric "!#%$& ' ()(sun-centred) +*-,. /0 +model *1!#&20in3his 4book 5 67 !#89 planets still on circular orbits centred on Sun advantages over the geocentric model more natural explanation of retrograde motion fewer epicycles (although still needs them!) explains changing brightness of planets over their orbits more accurate in predictions than the Ptolemaic system AST121 Lecture 3 AST121 5 Galileo Pointed the newly invented telescope at the sun, moon, and planets discovered: moon has a rough terrain - mountains, valleys, craters sun has spots - discovered that the sun rotates phases of Venus : implies Venus is in orbit about the Sun moons of Jupiter : 4 bright stars which orbit Jupiter : mini solar system Endorsed the Copernican system condemned by the Church and forced to recant AST121 Lecture 3 AST121 6 Tycho Tycho’s accurate data on Mars motion - accurate to 1 arcminute=1/60 of a degree both Ptolemaic and Copernican models were a relatively poor fit to the data Tycho wanted to know why and believed more accurate observations were needed compiled 20 years of accurate observations at his observatory Uraniborg AST121 Lecture 3 AST121 7 Kepler An astronomer-mathematician - proponent of Copernican system Analyzed Tycho’s data - hoped to make more accurate table for predicting planetary motions His great discovery was that planetary orbits are ellipses He also discovered empirical laws which describe planetary motions along these trajectories Kepler’s Three Laws 1. The planets move on elliptical orbits with the sun at one focus. 2. The motion along the ellipse is such that the line joining the sun to the planet sweeps out equal areas in equal times. 3. The square of orbital period of a planet is proportional to the semi-major axis of its ellipse cubed. We can derive the dimensions of the solar system from the 3rd law, ;=<>@?BACED"FHGJI CLK>NM OQPROQG AST121 Lecture 3 AST121 8 The Planetary Orbits The Inner Planets Mercury Venus Earth Mars The Outer Planets Jupiter Saturn Uranus Neptune Pluto AST121 Lecture 3 AST121 9