Astronomy: Greek Astronomy

I give here a summary of the contributions of the Greek philosophers that we know about through writings that were translated to Arabic and eventually re-translated to other languages long after the original writings perished. This account is undoubtedly patchy, and, we must assume, biased in unknown ways. The Greeks nevertheless laid down patterns of thought that we follow to this day. The Greek universe was an Earth-centered one, and the sun, moon, stars, and planets moved around the earth on great spheres.

Ptolemy's model for planetary motion, with deferents (big spheres) and epicycles (small spheres).

  • Proposed elementary laws of motion.
  • Affirmed spherical, stationary earth and heavenly spheres on which the sun, moon, and planets moved.
  • Laid down much of the basis for other branches of philosophical thought (e.g. logic, ethics).
  • Was influential in Europe in the middle ages.
  • Measured or estimated the sizes of earth, moon, and sun.
  • Measured relative earth-moon distance (25 earth diameters) and earth-sun distance (20 times the earth-moon distance).
  • Proposed a sun-centered cosmology (which was ignored).
  • Measured the absolute size of the earth.
  • This gave absolute scale to Aristarchus's size measurements.
  • Discovered "precession of the equinoxes".
  • Made position measurements of 1080 stars plus sun, moon, and planets. This catalog remained the best in existence until Tycho Brahe.
  • Invented the geometric devices used by Ptolemy because he realized that spheres-plus-uniform-motion did not reproduce the planetary position very well: the deferent/eccentric, equant, and the epicycle.
  • Compiled previous work into a 13-volume reference, the Almagest.
  • Refined and used Hipparchus's planet model to make tables of the future positions of the planets with errors usually less than 5 degrees.

Last modified: Mon Sep 11 23:02:38 CDT 2000