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About Lab 2 Retrograde Motion

This lab exercise lets you plot the position of Mars during a retrograde phase as observed from Earth. Observed coordinates are plotted on a graph of a portion of the celestial sphere.

You also have the opportunity to examine two different explanations for retrograde motion, one from a geocentric perspective, the other from a heliocentric perspective.

Retrorade motion is a FACT. It is what we observe the planets to do at certain times. The graph you will be plotting is taken from actual locations of Mars in its retrograde loop in 1970 - 71. Remember that the planets 'wander' with respect to the fixed star background, along with the sun and the moon. Most of the time the movements of all 7 bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn) are from east to west. The sun and the moon are good little wanderers. They always move from east to west. The planets are rascals.They mostly move westward except for those short time when they loop backward.

Retrograde motion is a fact (repeated for emphasis). It is the INTERPRETATION of the observed movements that we are concerned with.

There are two ways to intrepet the motion.

1) The planets move (along with the sun and the moon) around the earth in perfect circles at constant speed (and the same speed). To explain the retrograde loops it is necessary to add something like the spheres of Euxodos (early Greek interpretation) or the epicycles and other devices of Ptolemy (late Greek interpretation).

2) The planets move (along with the earth and the moon) around the sun and the retrograde motion is the result of earth catching up with and passing the slower moving outer planets (Mars, Jupiter, Saturn). I won't deal with the inner planets (Mercury, Venus) because it would only complicate things at this point.

Our focus is on WHY the geocentric explanation (1 above) continued to be favored even when it became obviously cumbersome. The answer lies in the paradigm that was created by Plato's question. Using the Pythagorean idea of the perfection of the circle and perfection of the heavens, he "insisted' on a circular explanation. The geocentric part (a separate paradigm) arose because that is what it looks like to us as we observe the heavens movements.

We will see how strong these TWO paradigms (geocentrism, circular perfection) were as we progress through the next few programs, and we will see what it took to finally change them.