Speculum
SPEK-yoo-luhm
Definition
A speculum — Latin for "mirror" — is a worked-out table that lists, for each birth planet, the sky-coordinate values you need before you can calculate primary directions. The standard columns are right ascension, declination, meridian distance (how far, in right ascension, the planet sits from the local meridian above or below it), and the day and night semi-arcs. Depending on which directional method you are using, a speculum may also carry oblique ascension, ascensional difference, polar elevation, and pole of position.
In Tradition
In the Renaissance-Latin tradition of primary directions, the speculum is the essential workbench: without it, you cannot calculate the arc of direction between any significator and promittor under any of the main methods — Ptolemaic, Placidean, Regiomontanan, or Morinus. A Placidean speculum needs each planet's semi-arcs and meridian distances; a Regiomontanan speculum needs the polar elevation of each planet's prime-vertical circle; the Morinus speculum is the simplest of the three.
In Practice
You build the speculum once, for the birth moment — by spherical trigonometry from right ascension, declination, and latitude, or with modern software — and then draw on it for every direction you calculate afterward. For each promittor-significator pair, the columns you need supply the values your chosen method requires. The speculum also speeds up scanning directions between planets: sort the planets by meridian distance and you can rank-order the directions coming up for any significator.
Historical Origin
The speculum has been standard in Renaissance-Latin primary-direction practice from Regiomontanus' Tabulae Directionum (1467) onward, with its standard columns settled by Placidus' Tabulae Primi Mobilis (1657) and Morinus' Astrologia Gallica (1661). For a modern treatment, Gansten's Primary Directions (2009), Chapter 4, reproduces standard speculum templates with worked examples.
Further Reading
- Bernhard Gansten, Primary Directions: Astrology's Old Master Technique
- Marion March & Joan McEvers, The Only Way to Learn Astrology Volume V & VI