Resources / Celestial timing
Decan Calculator
Identify the approximate solar and lunar decans for a selected date and time, including traditional decan ruler and zodiacal range.
Preparing the instrument for your time and place
The tradition
The decans divide the zodiac into thirty-six segments of ten degrees. They are Egyptian in origin, where a sequence of stars rising in succession through the night marked the hours and, across the year, the calendar.
Hellenistic and later Arabic astrology absorbed the scheme and assigned each decan a planetary ruler in the Chaldean order — the arrangement that gives the decans their alternative name, the faces. A planet in its own face carries a minor dignity: not the strength of rulership, but not nothing.
Reading the instrument
- The instrument reports the decan the Sun and the Moon occupy for the selected moment, with the zodiacal range and traditional ruler of each.
- Decan boundaries are exact tenths of a sign: 0°–10°, 10°–20°, 20°–30°. A body near a boundary is worth checking against a precise ephemeris.
- The ruler shown follows the Chaldean-order face attribution, which is the common traditional assignment — some sources use the triplicity-based scheme instead.
Where it descends from
- Egyptian decan lists
- Star tables from coffin lids and tomb ceilings, the earliest surviving form of the thirty-six-fold division.
- Picatrix and Agrippa
- The route by which decan imagery and planetary faces entered Latin magical practice.
What this does not claim
Solar and lunar positions are compact approximations intended for study and planning. Near a decan boundary, or where exact timing matters, consult a high-precision ephemeris.
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