ASTR 103 Lecture Notes - Lecture 3: Deferent And Epicycle, Semi-Major And Semi-Minor Axes, Orbital Period
Document Summary
Ancient astronomers invited geocentric models to explain planetary motions. Like the sun and moon, the planets were believed to move on the celestial sphere with respect to the background of stars. Most of the time a planet moves eastward in direct motion, in the same direction as the. Sun and moon, but frmo time to time it moves westward in retrograde motion. Ancient astronomers believed the earth to be the center of the universe; they invented a complex system of epicycles and deferents to explain the direct and retrograde motions of the planets on the celestial sphere. Nicolaus copernicus devised a comprehensive heliocentric model. Copernicus"s heliocentric (sun-centered) theory simplified the general explanation of planetary motions. In a heliocentric system, the earth is one of the planets orbiting the sun. The orbital sidereal period of a planet is measured with respect to the stars.
