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Planetary Motions The motions of planets in space and the reasons behind them Revolution Path of the planet around the sun How long does this take? 360º / year = ~ 1º / day Technically 365.25 days -- how do we account for this? Path is an ellipse Perhelion: closest (147 6 *10 km) on January 3 Aphelion: farthest (152 6 *10 km) on July 4 Rotation Why do we have day light and night time? What is the difference in Earth’s position relative to the sun? The Earth spins around its axis! An aside on Earth’s axis…… http://highered.mcgrawhill.com/sites/007299181x/student_view0/chapter2/sea sons_interactive.html How long does one rotation take? How would we measure this……..? Solar Days Solar day Measures time interval from noon on one day to noon on the next day Noon occurs when the sun is at the highest point in the sky 24 hours Sidereal Day Time for Earth to make one complete rotation w/ respect to a star OTHER THAN the sun (same spot in sky) 23 hours, 56 minutes, 4 seconds Direction relative to the star doesn’t significantly change because it is farther away BUT the sun is closer, so our direction changes ~ 1º / day It takes 4 more minutes to compensate for this, so solar day is 24 hours Earth Wobbles Precession Like a spinning top http://www.youtube.com/watch?v=hwY8zS4zGsg Earth does the same thing. Axis varies from 21.5º to 24.5º every 41,000 yrs Direction of axis pointing changes every 13,000 years (cycle is 26,000) Polaris vs. Vega Varying Orbit Earth’s elliptical orbit becomes stretched and condensed (more / less of an ellipse) over 100,000 to 400,000 years Why? Gravitational pull of other planets slightly pulls Earth What Governs this? Newton’s Law of Universal Gravitation M1M 2 F =G r2 Kepler’s Laws of Planetary Motion 1st Law: “The path of each planet about the sun is an ellipse with the sun at one focus” 2nd Law: “Each planet moves so that an imaginary line drawn from the sun to the planet sweeps out equal areas in equal periods of time” 3rd Law: T1 2 r1 3 ( ) =( ) T2 r2 In English, please….. Planets travel in an ellipse with the Sun at one focus; the other is symmetrically located at the opposite end of the ellipse Each planet revolves so that an imaginary line connecting it to the Sun sweeps equal areas in equal time intervals. This means it moves faster when it is nearer to the Sun. The length of time it takes a planet to orbit the Sun is related to the average distance from the Sun so that if you know its orbital period you can calculate its distance and vice versa. Consequences What must Kepler’s 2nd law mean in terms of a planet’s speed? Data Earth’s closest distance: (147 *106km) Earth’s farthest distance: (152 *106 km) Design / perform an experiment that proves Kepler’s 2nd Law to be true.