Primary directions
The oldest predictive technique — Ptolemy, Tetrabiblos III.10; systematised by Placidus (17th c.)
Ptolemy described directing points around the chart by the sky's daily rotation to time the events of a life, in the length-of-life material of Tetrabiblos III.10. Placidus de Titis built this into the proportional semi-arc method that carries his name. The rotation arc is turned into years by a 'key': Ptolemy's 1° = 1 year, or Naibod's key using the Sun's mean daily motion.
What it measures
The arc of primary (diurnal) rotation that carries a promissor to the place a significator held at birth, measured in the equatorial frame from true right ascension and declination — so a body's ecliptic latitude is honored — then converted to age by the time key.
Where practitioners disagree
- In mundo or in zodiaco; direct or converse; which key?
Placidus semi-arc in mundo (ours) or zodiacal directions; direct or also converse; keys of Ptolemy, Naibod, Cardan or Kepler
Converse directions and zodiacal directions are separate schools; the keys convert the same arc into different spans of years.
This engine: Direct mundane directions by Placidus semi-arc, Naibod key, with the Ptolemy key shown alongside.
Sources
- Claudius Ptolemy, Tetrabiblos III.10
- Martin Gansten, Primary Directions: Astrology's Old Master Technique (2009)