ASTR 170B2 Lecture Notes - Lecture 6: Deferent And Epicycle
13 Sep 2018
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Astr ptys 170b2 - lecture 6 - planetary orbits and gravity. Ptolemaic : epicycle made to explain the path of the planets (retrograde behavior) Best pre-telescope observer: determined positions and motion of planets. Each planet moves in an elliptical orbit around the sun at one focus. Same is true for any object in orbit. Elliptical: major axis is longer axis, minor axis is shorter one. Kepler"s second law: equal areas in equal time. An imaginary line joining the sun and a planet sweeps out equal areas if an ellipse over equal intervals of time. The planet moves faster the closer it gets to the sun. A^3=p^2 a= semi major axis p= period (time in years) Kepler"s laws are empirical- fitted to match observations. Kepler suspected that planetary motion might be caused by a force from the sun. Sir isaac newton (1643-1727) the law of the universal gravitation. Thought the moon is kept in orbit due to gravity.
