Summary Table of Solar System Models


This is a comparative, incomplete summary of three key models put forth by the Greek civilization (Ptolemy), the Renaissance (Copernicus), and a modern view based on precise measurements by Brahe (Kepler). These scientists all considered their key problem to be predicting the positions of the planets as viewed from the Earth.

Ptolemaic Copernican Keplarian

Fundamental Ideas The Heavens are perfect. The Heavens are perfect. The heavens are not perfect. The Sun is not a star. There is an underlying, unifying pattern. Observations Motions of the planets. Motions of the planets. Motions of the planets. Moon's phases and motions. Moon's phases and motions. Moon's phases and motions. Daily motion of the stars. Daily motion of the stars. Daily motion of the stars. Model Geocentric, with everything Heliocentric, with every- Heliocentric, with elliptical moving in perfect circular thing moving in perfect orbits and non-uniform orbits at uniform speed. circular orbits and speed. This required epicycles, uniform speed. More or little orbits around epicycles. larger orbits. Predictions Positions of the planets Positions of the planets Positions of the planets within the accuracy of within the accuracy of within the accuracy of observations of the time, observations of the time, observations of the time, ~2-5 o. ~2-5 o. ~10'-1 o. The Earth is still. The Earth itself moves. The Earth itself moves. Tests Failed to predict planets' Failed to predict planets' Predicted planets' positions positions to Brahe's positions to Brahe's (except Mercury) to modern accuracy. accuracy. accuracy. No stellar parallax, so Venus goes through all Newtonian laws of motion. the Earth doesn't move. phases, from new to full.Galileo observed the Sun Earth does move in space: and Moon to be imperfect. stellar parallax measured. Philosophy Earth is center of Sun, not Earth, is center Planets do not undergo Universe. of universe. natural motion. A force is required.

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