Astronomy - Basic Concepts: Retrograde Motion of the Planets - Scientific Revolution - Professor Robert A. Hatch
T H E   S C I E N T I F I C   R E V O L U T I O N
Dr Robert A. Hatch  -  University of Florida

Viewed from earth, planets in our solar system appear to undergo retrograde motion. Because we assume the earth moves, that is, that earth rotates daily and revolves around the sun each year, it follows that the apparent motion of the planets is the result of several motions.  Because we observe the planets from a moving platform, the apparent motion of the planets involves their motion as well as the motion of the earth.   But if we affirm our belief in the motion of the earth, we must also acknowledge that the motion of the earth is far from obvious. Historically, such a belief is a relatively recent curiosity.  One reason is that retrograde motion can be accounted for in a number of ways consistent with observation, common sense, and accurate prediction.  Below are two animated illustrations showing how the apparent retrograde motion of the planets can be described. The first illustration is a heliocentric (sun- centered) model, a view traditionally associated with Nicolas Copernicus (1473-1543).  The second illustration is a geocentric (earth-centered) model, traditionally associated with Claudius Ptolemy (fl. 140 AD).

Heliocentric (Sun-Centered) Model:    Retrograde Motion
Geocentric (Earth-Centered) Model:   Retrograde Motion

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