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The Motion of Mars: Part 3


This week let’s continue the astronomical tale of how the motion of Mars arguably sparked the origin of modern astronomy and science.  Last week we discussed how Claudius Ptolemy presented in about 150 AD a sophisticated mathematical model to explain why Mars and the other planets, moving around Earth, periodically moved retrograde relative to the background stars.  This Earth-centered model of the universe presented what was imagined to be a true representation of the motion of Mars and the rest of the planets for over a millennium.  As time went on, though, the model was not able to accurately predict where Mars should be in the sky and had to be mathematically “tweaked” over and over again in order to make it fit with observations.  Finally, in 1543, Nicolaus Copernicus presented what is generally considered a simpler explanation for the retrograde motion of Mars in the form of a Sun-centered model for the universe.  

With Mars and all the planets now orbiting the Sun, the retrograde motion of Mars is what we see from Earth as the faster-moving Earth passes the slower-moving Mars in their respective orbits.  So according to Copernicus, the retrograde motion of Mars is no longer actual motion, but an apparent motion due to our point of view from the Earth.  Copernicus’ Sun-centered model also included his correct order of the planets from the Sun and reasonable estimates of their orbital periods, but his model was not watershed as it also had some flaws that ultimately kept it from being widely accepted.

We will continue this astronomical tale from here next week. 


Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.

Dr. Todd Young hails from Minnesota and received his undergraduate degree in Physics & English from the University of Minnesota – Morris, his Master’s degree in Physics from Purdue University, and his Ph.D. from the University of Nebraska – Lincoln in Astrophysics. He has worked at Wayne State College since receiving his doctorate in 1998 and is currently a full professor of physics and astronomy. He teaches a variety of courses at Wayne State College, including university physics, astronomy, general education science, and astrophysics.
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