---
title: "Claudius Ptolemy"
type: "entity"
entity_kind: "person"
status: "hub"
canonical_name: "Claudius Ptolemy"
aliases: ["Ptolemy"]
first_seen: "2026-10-10T00:00:00.000Z"
writer_model: "claude-sonnet-5"
connects_to: ["[[entity-ptolemys-equant|equant]]","[[entity-johannes-kepler|Johannes Kepler]]","geocentric astronomy","Almagest","[[entity-tycho-brahe|Tycho Brahe]]"]
seek_code_commit: "d40ee39"
---


2nd-century CE astronomer and mathematician working in Alexandria whose *Almagest* systematized geocentric planetary models well enough that they stood, essentially unchallenged in their basic form, for roughly 1,400 years — longer than any other astronomical framework in history. His signature device, the **equant** — a point, distinct from the orbit's geometric center, about which a planet's angular speed is posited to be exactly uniform even though its distance from that point varies — is what let Ptolemaic astronomy fit observed planetary positions to within a fraction of a degree using only circles.

Matters to this vault as the historical pole a 2026 paper is built to argue *with*: [[claim-astronomia-ova-orbit-family-satisfies-keplers-second-law-and-exact-equant]] constructs an explicit orbit family satisfying both an exact Ptolemaic equant and Kepler's second law simultaneously — something neither astronomer's own model could do — and [[claim-ptolemy-equant-mars-error-8-arcminutes-tycho-precision-forced-kepler-to-abandon-it]] records the specific empirical gap (an ~8-arcminute error on Mars) that eventually forced the equant's abandonment.

## References

- [[claim-astronomia-ova-orbit-family-satisfies-keplers-second-law-and-exact-equant]]
- [[claim-ptolemy-equant-mars-error-8-arcminutes-tycho-precision-forced-kepler-to-abandon-it]]