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Ptolemy's Almagest and the geocentric model

This page delivers no verdict; it presents claims and evidence with their sources. No theory in science is absolute. Method

For everyone

Ptolemy's Almagest was a model placing the Earth at the centre and explaining planetary motion with nested circles (epicycles). For about 1,400 years it predicted planetary positions with usable accuracy. The work was translated into Arabic, and astronomers in the Islamic world, especially at the Maragha and Samarkand observatories, criticised and improved it.

For specialists

The model uses eccentric circles, epicycles and the "equant" point. Because the equant violated the principle of uniform circular motion, it was criticised by astronomers such as Naṣīr al-Dīn al-Ṭūsī and Ibn al-Shāṭir. The Ṭūsī couple is a geometric device producing linear oscillation from two circular motions. Ulugh Beg's Zīj (1437) gave new observed positions for about 1,000 stars.

Claims

The Earth rests motionless at the centre of the universe; the apparent motions of the planets are explained by nested circles.^[1]

Evidence

Supporting

Observation

The model could predict planetary positions close to the precision of naked-eye observations.^[1]

Challenging

Theoretical argument

Astronomers of the Maragha school argued that the equant was physically inconsistent and proposed alternative models.^[2]

Connections

Sources

BookToomer, G. J. (trans.) (1984). Ptolemy's Almagest. Duckworth, London; Springer, New York
BookSaliba, G. (2007). Islamic Science and the Making of the European Renaissance. MIT Press, Cambridge MA

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Cite this page

APA

Athena contributors. (2026). Ptolemy's Almagest and the geocentric model. Athena: The tree of human knowledge. Retrieved November 1, 2026, from https://latifkedi.github.io/athena/en/d/batlamyus/

BibTeX

@misc{athena_batlamyus,
  title        = {Ptolemy's Almagest and the geocentric model},
  author       = {{Athena contributors}},
  year         = {2026},
  howpublished = {\url{https://latifkedi.github.io/athena/en/d/batlamyus/}},
  note         = {Athena: The tree of human knowledge. Accessed: 2026-11-01}
}

Added: September 27, 2026 · Last changed: September 27, 2026 · Page history

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