Retrograde Motion

Katich, Rodney Bowen High School

Objectives 1) Students will learn the terms for objects in the heavens. 2) Demonstrate the basic motions of heavenly bodies. 3) Using models, show a variety of conceptual schemes explaining these motions. Equipment and Materials Model of Herodotus' Earth Sighting tubes Several spheres Three 50 ft. lengths of rope Flashlight Epicycle machine Recommended Strategies Identify objects in the skies such as the sun, moon, stars, planets, birds, planes, meteors, clouds, comets and so on. A model of each should be available. Ask students to order these in terms of distance from the earth: birds then clouds are pretty obvious. A discussion of the remaining objects requires some thought; eclipses suggest the correct sun-moon relationship (the sun is further). That the students have not observed the planets and thus have no observed basis for ordering these is a good point of departure for a short survey of historical views. Orientation on earth: the earth appears to be flat. The sun rises in the east and sets in the west providing a basic set of reference points. From these, north and south are derived. The sun is the first (as most prominent) celestial object considered. Its apparent motion can be demonstrated by following an imaginary sun at the end of one's finger from the east in an arc across the sky. Other aspects of the sun's activity are its southerly motion in the winter and returning more nearly overhead in the summer, and the length of day at these extremes. The moon is the next object to be considered; like the sun, it rises in the east, sets west, and can be observed to pass regularly through phases. These phases can be demonstrated using an eclipse model (using discs passing one before the other), and the correct model showing an angular relationship with three spheres and a flashlight. The stars, considered as a group, are also seen to rise east, set west, given the observer is facing south from a position in the northern hemisphere. Constellations (Ursa Major, Ursa Minor,etc.) rather than individual stars are tracked owing to the number of stars.. Once these observations have been established, various "explanatory" schemes are introduced. Plato's geocentrism can be contrasted with Aristarcus' heliocentric view. At this point one further observation is introduced: planets (or "wanderers") can be seen to move in a band of eight degrees on either side of the ecliptic; moreover, these planets seem to reverse directions from their observed motions to a retrograde motion for a period of time (during which they also appear brighter), before resuming their original path. The Platonic view has difficulty with these new data. Accordingly, Ptolemy modified Plato's view, retaining the geocentricity and circular motion while moving the geometric center of these circles from the Earth to points in space. This innovation can be demonstrated with an epicycle machine. Ptolemy's was the dominant view of planetary motion for nearly 1500 years, until Copernicus' observations led once again to a heliocentric (or sun centered) formulation. That this view can account for "retrograde motion" can be demonstrated by making a series of observations from one moving planet to another when both are in opposition to the sun. Thus both the apparent change in direction and the increased brightness "fit" a sun centered model. Heliocentrism, then, was revived and built upon with increasing precision with new observations made by Galileo (using a telescope) and culminating with Newton's Laws of Motion.
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