Fixed stars
Ptolemy's star natures, the medieval Behenian stars, and Vivian Robson (1923)
Ptolemy assigned planetary 'natures' to the constellations in Tetrabiblos I.9, and the medieval tradition singled out fifteen Behenian stars for magical and astrological use. Vivian Robson's The Fixed Stars and Constellations in Astrology (1923) is the standard modern compendium. The famous 'Royal Stars of Persia' are, on the evidence, an 18th-century French coinage — Dupuis named them in 1794 — that entered English astrology through R. H. Allen's Star Names (1899); the actual Avestan star-watchers were Sirius and Ursa Major, not the four stars usually listed.
What it measures
Conjunctions of a chart's points to individual named stars, by ecliptic longitude. Because the stars are fixed against the precessing tropical zodiac, their longitudes creep forward about 50 arcseconds a year, so each star's position is given for the chart's own date.
Where practitioners disagree
- Which nature — the star's own, or its constellation's?
The traditional planetary nature per star (largely Robson's) — or Ptolemy's nature for the whole constellation
Ptolemy assigned natures to constellations; the star-by-star equations (e.g. Algol = Saturn/Jupiter) are later inferences that merge his text with the Almagest's positional catalogue.
This engine: The traditional per-star nature, at a 1° conjunction orb.
Sources
- Vivian E. Robson, The Fixed Stars and Constellations in Astrology (1923)