Arc of Direction

ark uhv dih-REK-shuhn

Definition

An arc of direction is the angular gap — measured in degrees of right ascension — between a significator and a promittor as the sky's daily turning carries them across your local sphere. You convert that arc into years of life with a chosen time-key: the Ptolemaic key at 1 degree per year, the Naibod key at about 0.9856 degrees per year, or solar arc, 1 degree per year tied to the Sun's average motion. The arc itself is read off the speculum using whichever directional method you have chosen — Ptolemaic, Placidean, Regiomontanan, or Morinus.

In Tradition

In the Renaissance-Latin tradition of primary directions, the arc of direction is the main result the whole calculation is after: it answers, in a number, the question "when does this directional event come due?" The same significator-promittor pair gives different arcs under different methods, because each method measures the daily motion against a different reference circle — Placidean uses the planet's own semi-arc, Regiomontanan the prime vertical, Morinus the equator, and Campanus the prime vertical divided into equal parts.

In Practice

You name the significator — the Ascendant, Midheaven, Sun, Moon, or a Lot — and the promittor — a birth planet, or an aspect-degree to one — then apply your chosen method to work out the arc of direction. Converting that arc to years with your time-key gives the age at which the direction comes due. A direct direction carries the promittor forward, in the sky's natural turning, toward the significator; a converse direction does the reverse, carrying the significator back toward the promittor against that turning. You read each direction in light of the natures of the significator, the promittor, and the aspect between them.

Historical Origin

Calculating the arc of release is a cornerstone of Ptolemy's Tetrabiblos III.10-15 and IV (2nd century CE), and was systematized in the Latin tradition by Regiomontanus' Tabulae Directionum (1467), Placidus' Tabulae Primi Mobilis (1657), and Morinus' Astrologia Gallica (1661). For modern treatments, see Gansten's Primary Directions (2009) and March and McEvers' The Only Way to Learn Astrology, volumes 5 and 6.

Further Reading

  • Claudius Ptolemy, Tetrabiblos
  • Bernhard Gansten, Primary Directions: Astrology's Old Master Technique
  • Marion March & Joan McEvers, The Only Way to Learn Astrology Volume V & VI