Decan
A decan is a division of each zodiacal sign into three sub-periods of 10 degrees, making 36 decans for the entire zodiac. Each decan is given a planetary sub-ruler that nuances and refines the expression of the main sign.
Origin and etymology
The term decan comes from the Greek dekanos, derived from deka (ten). The system originates in Egyptian astronomy of the third millennium BCE: the Egyptians identified 36 stars or groups of stars, each rising at a specific moment of the year and used to measure the hours of the night. The first decanal diagrams appear on the coffin lids of the Twelfth Dynasty (around 2000 BCE). The system was integrated into Hellenistic astrology by Teucer of Babylon (first century), then reformulated in the Hermetica. Firmicus Maternus, in the fourth century, gave it a complete Latin presentation in the Mathesis.
Evolution and tradition
Two main attributions coexist. The Chaldean distribution, codified by Ptolemy, matches the decans to the seven traditional planets in the order of their geocentric speed: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. The triplicity distribution, popular in modern astrology, assigns each decan a planet of the same triplicity (element). Thus Leo (a fire element) sees its three decans governed by the Sun, Jupiter, and Mars. In the twentieth century, the occultism of the Golden Dawn and Aleister Crowley linked the 36 decans to the 36 numbered minor cards (from 2 to 10), a framework taken up in the Thoth Tarot.
Practical use
To locate your decan, identify the exact position of your natal Sun in degrees within the sign. If you are Aries 5°, you belong to the first decan, ruled by Mars (the sign's ruling planet). Aries 15° falls in the second decan (Sun in triplicity) and Aries 25° in the third (Jupiter). This precision refines the reading: a first-decan Leo, doubly solar, is read as more radiant; a second-decan Leo, under Jupiterian influence, as more expansive. On Tarotoui, applying the 36 decans to the minor cards allows for a fine astrological reading of your spread.
Going further
The two systems — Chaldean and triplicity — are not equivalent and sometimes give contradictory results. The Hellenistic tradition (Vettius Valens, Dorotheus of Sidon) also knew of facies decans tied to symbolic images, rediscovered in the 1990s by traditional astrology (Robert Hand, Chris Brennan). The iconography of the decans informed Renaissance art, notably the famous fresco cycle of the Palazzo Schifanoia in Ferrara (1469-1470).