Retrograde motion of mars ptolemy biography wikipedia

Ptolemy to the Rescue? In order to preserve the geocentric cosmology of the time and to account for retrograde motion of Mars, Ptolemy had to make a model of planetary motion that invoked the use of epicycles. An epicycle is basically a little "wheel" that orbits on a bigger wheel.

Retrograde orbit

The use of epicycles as a desperate attempt to preserve geocentric cosmology makes the orbits of planets very complicated and violates the scientific search for simplicity. Visualization of epicycles Epicycles and Planetary Motion In the nutso model of Ptolemy, planets were attached, not to the concentric spheres themselves, but two circles attached to the concentric spheres, as illustrated in the adjacent diagram.

These circles were called "Epicycles" , and the concentric spheres to which they were attached were termed the "Deferents". Then, the centers of the epicycles executed uniform circular motion as they went around the deferent at uniform angular velocity, and at the same time the epicyles to which the planets were attached executed their own uniform circular motion.

The net effect was as illustrated in the following animation. As the center of the epicycle moves around the deferent at constant angular velocity, the planet moves around the epicycle, also at constant angular velocity. The apparent position of the planet on the celestial sphere at each time is indicated by the line drawn from the earth through the planet and projected onto the celestial sphere.

The resulting apparent path against the background stars is indicated by the blue line.