GEOCENTRIC UNIVERSE VS. HELIOCENTRIC UNIVERSE : Refer to: The Origin of Modern Astronomy (Pg. 32-39) Retrograde motion: the apparent backward motion of a planet as seen from a vantage point From the Earth as a vantage point, the picture shows how, the planet further from the sun seems like it is moving in a backwards loop. PTOLEMY'S MODEL (140AD): "Earth-centered" or "geocentric" Ptolemy suggested that celestial objects — including the planets, Sun, Moon, and stars — orbited Earth. COPERNICUS’ MODEL (1542 AD): "Sun-centered" or "heliocentric" * explains retrograde motion through epicycles in which planets move in loops backwards to forwards around a Copernicus thought that the planets orbited the Sun, and that larger circle called a deferent.The deferent was centered the Moon orbited Earth. Since the Sun was the center of the around a point halfway between the Earth and another universe, it did not move, nor did the stars. point called the equant . *explains retrograde KEY: motion by planets passing Epicycle : smaller circle each other . The planets Deferent : larger circle with smaller orbits, or Equant : larger black dot closer to the Sun, move faster, so they overlap the outer and larger planets; eventually the planets catch up to each other. Modern Day Observations: -The Sun is not in the center of the universe [it is the center of the solar system ], and it does moves as well as other stars. -Copernicus and Ptolemy thought the orbits of the planets were circular, but we now know they are elliptical. NOTE: The outer planets, like Uranus and Neptune, are missing from both charts because they had not been discovered at the time of Ptolemy and Copernicus. The planets are lined up to make the charts easy to read; they never align this way in nature. Sources: Astro by Backman/Seeds http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/g37/ http://cseligman.com/text/sky/retrograde.htm http://en.wikipedia.org/wiki/Deferent_and_epicycle http://en.wikipedia.org/wiki/Apparent_retrograde_motion ;http://1.bp.blogspot.com/_XAakLKI3wRs/TQmZlO4cGmI/AAAAAAAAAGo/mYxg8JEU1e8/s1600/retrograde-motion.jpg .