Expert: Steven LJ Russo - 12/4/2009

Question
Good evening Sir

In a post on 11/25 you discussed geocentricism and heliocentric.  

If we compare the Ptolemaic and Copernican models of the universe how they are alike or different? What evidence did each use to support their ideas?  How did each explain the retrograde motion of the inner planets?

My thoughts are based on the following:

Claudius Ptolemy attempted to explain the motion of the planets mathematically by devising a small circle rotating along the edge of a larger circle that enclosed a slightly off-center Earth.  In these ways he weakened the principles of geocentricism and uniform circular motion.  

Ptolemy gave mathematical form to Aristole’s model in the Almagest. Ptolemy preserved the principles of geocentricism and uniform circular motion, but he added a small circle referred to as an epicycle, which slid around a deferent, a larger circle that surrounded the Earth.  By adjusting the size and rate of rotation of the circles, he could approximate the retrograde motion of a planet.  

To adjust the speed of the planet, Ptolemy supposed that the Earth was slightly off center and that the center of the epicycle moved such that it appeared to move at a constant rate as seen from the point called equant.

Nicolaus Copernicus studied the Ptolemaic universe.  Copernican proposed that the universe was heliocentric, or sun centered.  He proposed that the Earth rotated on its axis and revolved around the sun.  In the Copernican system, the Earth moves faster along its orbit than the planets that lie farther from the sun.  Consequently, the Earth periodically overtakes and passes these planets.  

Copernicus could explain retrograde motion without epicycles.  However, he could not predict the positions of the planets any more accurately than the Ptolemaic system could.  Copernicus called eqaunts “monstrous” in that they violated both geocentricism and uniform circular motion.  In devising his model, Copernicus returned to a strong belief in uniform circular motion.  

Even after researching the aforementioned I still have no clue HOW
to explain the retrograde motion of inner planets.  Does that mean that the planets rotate in opposite directions?  

As a child I enjoyed studying the solar system, but I lacked the full scope of space and/or astronomy.  I am learning a lot and I am more observant when I look into the nighttime sky ... (before I just saw mortars).  Nevertheless, I applaud you and I appreciate your willingness to assist those of us that are lacking knowledge in physical science.  

Answer
Actually, retrograde motion is easier than you think.

Imagine you are in a car going 70 mph.  You pass a car that is going 40 mph in the same direction as you.  From your point of view the car you pass looks like its going backwards.

That's retrograde!

Steve