- 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.