Physics · Ch 6 — Gravitation
Heliocentric System over Geocentric System
Heliocentric System over Geocentric System
When the planets are tracked against the background stars over several months, most of them are seen to drift steadily eastward -- but every so often, a planet like Mars appears to slow down, reverse direction, loop backward (westward) for a few months, and then resume its normal eastward drift. This puzzling backward loop is called retrograde motion.
Ptolemy's fix: epicycles. In the geocentric model, Aristotle and Ptolemy assumed planets moved on circular orbits around the Earth -- but a simple circular orbit around the Earth cannot explain a temporary reversal in direction. Ptolemy's workaround was the epicycle: he proposed that while a planet orbited the Earth on a large circular path (the deferent), it also traced out a smaller circular loop (the epicycle) superimposed on that path. The right combination of the two circular motions could reproduce the observed retrograde loop. It worked mathematically, but every newly observed planet needed its own carefully-tuned epicycle, and the model grew steadily more complicated and artificial. …
What this figure shows. A month-by-month track of Mars's position against the background stars is plotted from February through December. The path moves steadily eastward from February to June, then visibly reverses and loops backward (westward) through July, August and September, before turning around again and resuming its eastward drift from October onward -- this observed backward loop is the 'retrograde motion' that any correct model of the so …
What this figure shows. The Sun is fixed at the centre with both the Earth's and Mars's near-circular orbits drawn around it. Because Earth orbits faster and on an inner track, a sequence of Earth-Mars sighting lines (labelled a through g, one per month) is drawn from Earth's changing position toward Mars; as Earth overtakes and passes Mars on the inside track, these sighting lines swing backward for a few months before swinging forward again, reproducing the observed retrograde loop purely as an effect of relative motion, with no need for Ptol …