PHIL 160D3 Lecture 17: Copernican System, Realism, Instrumentalism
University of Arizona
The Copernican System, Realism Instrumentalism Copernican System Movement of the stars The stars motion is apparent Stars are motionless The Earth rotates on its axis Why is there a stellar sphere then? Influence of Aristotelian worldview Movement of the Sun The sun appears to rise and set over the course of 24 hours Earth rotates on its axis every 24 hours The sun appears to move through the sky over the course of a year The Earth orbits the sun every year Movement of the planets Daily motion with the stars Distinguishing planets from stars: Planets move slowly through the stars, Retrograde motion, planets are brighter when in retrograde The planets appear to move with the stars The Earth rotates on its axis The planets appear to move through the stars seasonally: due to apparent and genuine motion, the planets and Earth both orbit the sun Copernicus epicycles dont explain retrograde motion (he doesnt need them to make the explanation) Compare: Venus Proximity to the Sun Both the Copernican and Ptolemaic models accommodate the fact that Mercury and Venus always appear close to the Sun The Copernican model predicts this fact. It is a consequence of the core model itself. The Ptolemaic model does not predict this fact: it requires extra assumptions about the alignment of the planets epicycles; the alignment is a coincidence. Problems Conflicts with available data that supports geocentrism Little motivation to change. Not simpler or more empirically accurate than Ptolemaic system Result: Copernican model regarded as a tool for making predictions. Not an accurate description of the world.