Astronomy - Basic Concepts - Retrograde Motion - Scientific Revolution - Professor Robert A. Hatch
RETROGRADE  MOTION  -  HELIOCENTRIC
Professor Robert A. Hatch  -  University of Florida

 
The animated illustration above represents retrograde motion from a heliocentric (sun- centered) perspective.  Here the sun is shown in the center of two orbits, the inner orbit representing earth, the outer orbit a superior planet.  Because the earth orbits the sun faster than the outer planets (Mars, Jupiter, Saturn) the apparent position of those superior planets, viewed against the backdrop of the 'fixed stars', appears to undergo a 'looping' retrograde motion.  Put differently, when a superior planet 'retrogrades' it appears first to move in its direct order, it then slows down and appears to stop (stationary point one).  The planet then reverses its direction (undergoes retrograde motion).  After reversing direction, it again slows down and appears to stop a second time (stationary point two).  Finally, the planet reverses a second time and continues in its direct order.  From an observational standpoint, when the two planets are in opposition to each other (on opposite sides of the sun) the outer planet cannot be seen from earth.  From a heliocentric perspective, then, retrograde motion is an illusion.  The appearance results from observations made on a moving platform (earth) which 'laps' or overtakes the outer, slower moving planets.  But this interpretation is only one of several possibilities.  The motion of the earth is not at all obvious to the observer.  In retrospect, the solution to the problem of retrograde motion was not simply to describe the motion by means of geometry but to explain it by means of cosmology.  The consequence of this simple sleight of hand (understood in geometrical terms) brought a profound shift of categories (understood in cosmological terms).  The Copernican Model (arguably a lucky guess) was a Trojan Horse.  It ushered in a new set of categories about reality (space, time, matter, cause) and new criteria for judging claims to truth (to distinguish real and apparent).


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