1. Chapter 5 Planetary Motion • Historical Description of Planetary Motion • Greek Philosophers’ model (300 B.C) • Earth is at the center of the Universe • Sun, Moon, Stars are fixed in concentric spheres revolving around Earth • Planets have retrograde motion • Planets appear to move backwards sometimes • Ptolemy (200 A.D.) explains with Epicycles • Circles not spheres for sun, moon, planets • Smaller circular motions on the larger orbit • Explained retrograde motion

2. Copernicus and Galileo: Heliocentric Solar System • Sun is center of Solar System, planets including Earth, revolve around sun • Explains retrograde motion, plus other of Galileo’s observations • Used circles for the orbits: didn’t work for everything • Kepler’s Planetary Motion (1571-1630) • Assistant to Tycho Brahe, who had the best observations yet of the sky • Devised 3 laws from the data: Testable Rules or Empirical Laws • Planetary orbits are ellipses, not circles • Orbital radius sweeps out equal areas in equal times

3. These 3 laws accurately predicted the motions of the planets • As Empirical Laws, they still needed to be explained • Newton’s Universal Gravitation • Newton realized planetary motion was like projectile motion • Orbit of a planet is just falling from very high • If you threw a ball fast enough from a high enough altitude, it would orbit Earth • The moon is a big falling rock!!! • Universal Gravitation • W = mg, so mass must be involved • Gravity is inversely proportional to distance

4. 3) Force of Gravity can be calculated • G = universal gravitational constant = 6.67 x 10-11Nm2/kg2 • G is very small, so m1 or m2 must be large for us to notice F • Weight and Gravity • Earth is very large, so its attraction force is very large mE = 5.98 x 1024 kg • Radius of the earth: rE = 6.37 x 106 m • W = FG • Calculate W for 50 kg person 2 different ways Cavendish, 1731-1810 W = mg

5. Moon and Satellite motion • Moon orbits Earth according to Kepler’s Laws = Satellite • Phases of the moon • Full • Half • New • Eclipses • Lunar: Earth’s shadow covers the moon • Solar: Moon blocks the sun from the Earth • Tides: Moon’s Gravitation affects oceans, creating bulges • High Tides: closest and farthest from the moon • Low Tides: midway from the moon

6. Other Satellites • Anything in orbit of the Earth is a satellite: many man-made objects • They obey Kepler’s Laws: • We can manipulate r and T to get any distance or period we want • Sychronous orbit: T = 24 h, satellite stays over same place all day • Calculate r for a synchronous = 42,000 km (7 times re) • Make r shorter, T becomes shorter • Make T shorter, r becomes shorter