display | more...
In astronomy, retrograde motion is the apparent reversal of motion observed in outer planets as seen from Earth. As a planet moves across the sky relative to the fixed stars, it tends to move eastward, or in a clockwise direction as seen from the north pole. Due to the Earth's own motion along a shorter orbit, the planet will appear to reverse direction and move westward. The earth has a shorter revolultion, so it sort of 'cuts inside' the outer planet. The Earth's motion changes our point of view, leading ito the appearance of a direction reversal. Before Kepler, religious purists who preferred an Earth centered cosmology used to say these were the "planets bowing to Man, God's greatest creation."

Past Cosmologists, including Ptolemy and Copernicus assumed a circular orbit. But a pure circular orbit does not explain retrograde motion. So both the Earth-centered ideas of Ptolemy and the heliocentric model fo Copernicus needed the planets to peform a series of epicycles, or small circles in order to accurately predict planetary motion. The epicycles disappeared when Johannes Kepler, correctly deduced that planets move in elliptical orbits.

Log in or register to write something here or to contact authors.