---
title: "Johannes Kepler"
type: "entity"
entity_kind: "person"
status: "hub"
canonical_name: "Johannes Kepler"
aliases: ["Kepler"]
first_seen: "2026-10-10T00:00:00.000Z"
writer_model: "claude-sonnet-5"
connects_to: ["[[entity-ptolemys-equant|equant]]","[[entity-tycho-brahe|Tycho Brahe]]","[[entity-claudius-ptolemy|Claudius Ptolemy]]","elliptical orbits","Astronomia Nova"]
seek_code_commit: "d40ee39"
---


German mathematician and astronomer (1571–1630) who, working from [[entity-tycho-brahe|Tycho Brahe]]'s unprecedentedly precise Mars observations, derived the three laws of planetary motion — orbits are ellipses with the Sun at one focus, equal areas are swept in equal time, and a planet's orbital period relates to its orbital distance by a fixed power law — replacing circle-and-equant astronomy with a dynamical account later shown (by Newton) to follow from gravity.

Matters to this vault as the figure whose own working papers and whose laws anchor a 2026 paper's central question: [[claim-astronomia-ova-orbit-family-satisfies-keplers-second-law-and-exact-equant]] asks whether an orbit can satisfy Kepler's second law exactly at one point while also having a true [[entity-ptolemys-equant|equant]] at another; [[claim-kepler-1609-astronomia-nova-rejected-puffy-cheeked-via-buccosa-oval-orbit]] records his own 1609 *Astronomia Nova* documenting the trial-and-error — including a near-elliptical "puffy-cheeked" orbit he explicitly rejected — on the way to the ellipse; [[claim-ptolemy-equant-mars-error-8-arcminutes-tycho-precision-forced-kepler-to-abandon-it]] records the specific Mars data that forced him to abandon Ptolemy's equant rather than reject it on philosophical grounds alone.

## References

- [[claim-astronomia-ova-orbit-family-satisfies-keplers-second-law-and-exact-equant]]
- [[claim-kepler-1609-astronomia-nova-rejected-puffy-cheeked-via-buccosa-oval-orbit]]
- [[claim-ptolemy-equant-mars-error-8-arcminutes-tycho-precision-forced-kepler-to-abandon-it]]
- [[claim-bracco-provost-2009-contains-no-8-arcminute-mars-figure-astronomia-ova-cites-it-for]]
- [[claim-8-arcminute-mars-figure-traces-to-kepler-astronomia-nova-ch19-not-bracco-provost]]

## Updates

- 2026-10-13: Astronomia Ova cites Bracco & Provost (2009) for its ~8-arcminute Mars equant-error figure, but a direct read of Bracco & Provost finds no such figure there at all. The number is genuinely Kepler's own — his *Astronomia Nova* (1609), Ch. 19, credits Tycho Brahe's observations for deducing "this eight-minute error of Ptolemy's in regard to Mars" — so the true root of a figure a 2026 paper attributes to a 2009 paper turns out to be Kepler's own 17th-century text, not yet read by this vault above Tier 3.
  ([[claim-8-arcminute-mars-figure-traces-to-kepler-astronomia-nova-ch19-not-bracco-provost]], [[claim-bracco-provost-2009-contains-no-8-arcminute-mars-figure-astronomia-ova-cites-it-for]])