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Announcements • Homework 3 due Monday • Test next week: Wednesday or Thursday (your choice); at SL 228 testing center; one hour time limit; no calculators; study homework, lectures, textbook Kepler’s Laws 15 September 2006 Today: • How Johannes Kepler found a much more accurate scheme for predicting planetary positions, based on new mathematical laws Tycho Brahe Danish astronomer, 1546 - 1601 Tycho made highly accurate (within 1/60 degree) nakedeye observations of the positions of stars and planets over many years. He proposed a compromise model in which all the other planets orbit the sun, but the sun orbits the earth. Johannes Kepler German astronomer, 1571 - 1630 Kepler worked under Tycho during 1600 - 1601, then inherited the records of Tycho’s detailed observations. He accepted the Copernican hypothesis, but abandoned the assumption that all motions are circular. The Orbit of Mars based on Tycho’s observations Toward Mars Earth The Orbit of Mars Earth 1.9 years later The Orbit of Mars The Orbit is an Ellipse! Focus Focus The sun is at one focus; there’s nothing at the other. Planets go faster when they’re close to the sun A line drawn from the sun to the planet sweeps out equal areas in equal times. Outer planets move slower than inner planets. Time (yr) Radius (AU) Mercury 0.24 0.39 Venus 0.61 0.72 Earth 1.00 1.00 Mars 1.88 1.52 Jupiter 11.86 5.20 Saturn 29.46 9.54 Outer planets move slower than inner planets. Time (yr) Radius (AU) Mercury 0.24 0.39 Venus 0.61 0.72 Earth 1.00 1.00 Mars 1.88 1.52 Jupiter 11.86 5.20 Saturn 29.46 9.54 (Time)2 (Radius)3 0.37 0.37 Outer planets move slower than inner planets. Time (yr) Radius (AU) (Time)2 (Radius)3 Mercury 0.24 0.39 0.058 0.059 Venus 0.61 0.72 0.37 0.37 Earth 1.00 1.00 1.00 1.00 Mars 1.88 1.52 3.53 3.51 Jupiter 11.86 5.20 140.7 140.6 Saturn 29.46 9.54 867.9 868.3 (Time in years)2 = (Radius in AU)3 Kepler’s Laws 1. Orbits are ellipses, with sun at one focus. 2. Equal areas in equal times (faster when close to sun) 3. (Time in years)2 = (Radius in AU)3 Bottom line: The universe speaks math!
