Johannes Kepler

yoh-HAH-nuhs KEP-ler

Definition

Johannes Kepler (1571–1630) was a German mathematician, astronomer, and working astrologer. He assisted Tycho Brahe and then served as Imperial Mathematician at the court of Rudolf II in Prague. You may know him for the three laws of planetary motion; he also computed the Rudolphine Tables (Ulm 1627) — the first reasonably accurate astronomical tables, built on Tycho's observations — and wrote a substantial body of astrology, including chart readings for the imperial family and for Count Wallenstein.

In Tradition

Histories of astrology treat Kepler as both a major reformer and a witness to early-modern practice. Holden records that he was the first astronomer to recognise that the planets move at varying speeds along elliptical orbits, rather than at constant speed in perfect circles. He also introduced the minor aspects — additional angles between planets beyond the classical five — of which the 30°, 45°, 135°, and 150° aspects are still in use today.

In Practice

Kepler practised astrology actively, casting natal charts and forecasts for Rudolf II, his family, and Wallenstein. His tracts document this work: *De fundamentis astrologiae certioribus* (Prague 1602), *Judicium de trigono igneo* (1603), *De Stella nova in pede Serpentarii* (Prague 1606), and *De cometis libelli tres* (Augsburg 1619). The astrology he defended was not the whole inherited tradition — he rejected the houses, set aside much medieval doctrine, and tried to ground prediction in a geometric-musical theory of aspects, worked out in *Harmonices Mundi* (1619) Book IV. *Tertius Interveniens* (1610) defended what he saw as the rational core of astrology against a contemporary critic. Today astrologers meet Kepler mainly through his minor aspects and through John Addey's 20th-century revival of harmonic theory, which grew from Kepler's geometric programme.

Historical Origin

Kepler was born in Weil der Stadt in 1571 and died in Regensburg in 1630. Holden documents his posts as assistant to Tycho Brahe and as Imperial Mathematician under Rudolf II from 1601, with astrological work for Wallenstein into the 1620s. His astrological tracts (1602, 1603, 1606, 1619) and his theoretical works — *Harmonices Mundi* (1619) and *Tertius Interveniens* (1610) — are public-domain originals, in facsimile and selective English translation. The Rudolphine Tables (Ulm 1627) embed his elliptical-orbit theory in usable form. Holden's view that Kepler anticipates secondary progressions on the day-for-a-year measure rests on Kepler's own writings (Holden 2006, p. 182).

Further Reading

  • Johannes Kepler, Harmonices Mundi (1619; Book IV on aspect theory)
  • Johannes Kepler, Tertius Interveniens (1610)
  • James H. Holden, A History of Horoscopic Astrology