The Almagest is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy (AD 100 - 170).
One of the most influential scientific texts of all time, it canonized a geocentric model of the Universe that was accepted for more than 1200 years from its origin in Hellenistic Alexandria, in the medieval Byzantine and Islamic worlds, and in Western Europe through the Middle Ages and early Renaissance until Copernicus.
It is also a key source of information about ancient Greek astronomy.
Ptolemy's cosmos
The cosmology of the Syntaxis includes five main points, each of which is the subject of a chapter in Book I.
What follows is a close paraphrase of Ptolemy's own words from Toomer's translation.
- The celestial realm is spherical, and moves as a sphere.
- The Earth is a sphere.
- The Earth is at the center of the cosmos.
- The Earth, in relation to the distance of the fixed stars, has no appreciable size and must be treated as a mathematical point.
- The Earth does not move.
Ptolemy assigned the following order to the planetary spheres, beginning with the innermost:
- Moon
- Mercury
- Venus
- Sun
- Mars
- Jupiter
- Saturn
- Sphere of fixed stars