Reading Questions: Claudius Ptolemy, Almagest I, 3-4, 6-7
(Questions for reflection, which cannot necessarily be answered simply by reading the text but should be provoked by it, are marked with an asterisk.)
0. When did Ptolemy live? Where was his major work published? What was its significance for the middle ages?
1. According to Ptolemy, what was the main phenomenon that led ancient astronomers to the idea of a sphere?
2. Why, according to Ptolemy, did the ancients stick with the notion once it had been proposed? Does he seem to think this is reasonable? Does he himself accept it?
3. Ptolemy gives two arguments against the notion that the stars move off in a straight line. What are they?
4. Ptolemy clearly has contempt for the "igniting and extinguishing" theory of the rising and setting of stars. What is his argument against this? What is it based on? (Faith? Scripture? Authority? Pure reason? or something else?)
5. In the paragraph beginning "Finally, to assume any motion ..." Ptolemy argues against the notion that the distances of the stars may vary. On what observational fact is this based? Try to reconstruct the geometric idea behind his argument, taking (as he would have) the earth to be fixed in place while the heavens wheel around it.
6. Objects at the horizon often appear larger to us. What is the explanation Ptolemy gives for this? Can you make sense of it in modern terms despite the odd vocabulary? What analogy does he offer for the phenomenon?
7. What is Ptolemy's principal argument for a spherical shape? How does it differ from Aristotle's argument in reading 1.8 from our anthology (in the section about the earth’s being of no great size)?
8. How does Ptolemy refute each of the alternatives he considers? Try to draw a picture that shows the geometric features he is talking about.
9. What additional argument does Ptolemy offer for the curvature of the earth's surface?
10. How large is the earth, according to Ptolemy, compared to the distance of the sphere of the fixed stars? Why are observations at different latitudes relevant to this? (Hint: assume that the stars are no very great distance above the earth, then consider the effects of a drastic change of latitude.)
11. What is Ptolemy's point about the horizon bisecting the sphere of the stars? Try to illustrate this by drawing a diagram, making the surface of the sphere of the stars very near to the earth to see what difference this would make to the proportion of the heavens visible to someone on the surface of the earth.
12. Lay out Ptolemy's first argument against the motion of the earth, being sure to give the key Aristotelian premise. Which premise is underwritten by everyday observation?
13. Ptolemy notes the objection that the earth has great weight but is not, in his system, supported by anything. What is his response?
14. What does the comment about the earth's being "pressed in equally" mean? Does it seem to indicate a plenist or vacuist conception of the universe?
15. "These heavy bodies..." What conclusion is Ptolemy arguing for here? Note the resemblance to the premises of Aristotle's argument (in our anthology, from reading 1.8: “If there were a similar movement from each quarter of the extremity to the single center ...”). Are the conclusions the same? Are they compatible?
16. What, according to Ptolemy, provides the decisive refutation of the notion of a rotating earth -- astronomical observation or physical phenomena?
*17. Try to express Ptolemy's objection as an argument, filling in any unstated but required premises as you go. What Aristotelian premise regarding motion and force seems to be presupposed?