Decanates

Definition

Decanates are the three 10-degree thirds of a zodiac sign. Two distinct lord-assignment schemes coexist in the tradition: the Chaldean-order face scheme (Greek-Persian consensus per Al-Biruni) which assigns the seven planets in their descending order beginning with Mars on the first face of Aries, and the alternate-lord scheme (Sanskrit drekkāṇa, Persian darigan / Drikan) reported by Al-Biruni from the Indian tradition, which assigns the first decanate to the lord of the whole sign, the second to the lord of the fifth sign from it, and the third to the lord of the ninth sign.

In Tradition

Across Hellenistic, Arabic-Persian, and Western tradition the decanate division is treated as one of the standard sub-sign frameworks, alongside half-signs, terms (bounds), and Ptolemy's sign-thirds. The Chaldean-order face is the dominant scheme in the Hellenistic-Western lineage and is the basis of the face essential-dignity; the Hindu drekkāṇa scheme is preserved through Al-Biruni's comparative cataloguing and circulates as the modern 'decanate' in some popular Western sources.

In Practice

Astrologers identify the relevant decanate by dividing the sign into 0°-9°59′ (first), 10°-19°59′ (second), and 20°-29°59′ (third), then read the corresponding lord. In Chaldean-order practice (face), the face-lord at the planet's exact degree contributes the weakest essential-dignity score (+1 in Lilly's tally) and is consulted for descriptive detail in horary. In the Hindu drekkāṇa-style alternate-lord practice, each decanate carries a planet linked to the whole-sign lord by trine — the same-element relationship that 'decanate ruled by a sign of the same element' formula encodes. Modern Western sources sometimes present the decanates as same-element-by-sign (e.g. Aries decanates as Aries-Leo-Sagittarius), which corresponds to the Hindu trine-based scheme rather than the Chaldean-order face scheme.

Historical Origin

Al-Biruni's *Kitāb al-Tafhīm* §§448-452 (c. 1029) catalogs the half-signs, faces, figures, decanates, and Ptolemy's sign-thirds side by side, with §451 distinguishing the Hindu darigan / Drikan scheme from the Persian-Greek face scheme of §449. The 10°-third division itself originates in Egyptian decanal star tables (ca. 2400 BCE onwards) tracking heliacally rising asterisms; the planetary-ruler overlay in Chaldean order is a Hellenistic-era development attested in Dorotheus, Valens, and Firmicus Maternus.

Etymology

Origin: Greek. Meaning: From δεκανός (dekanos, 'ten-degree division'), built on δέκα (deka, 'ten') — the technical name in the Hellenistic tradition for each 10° sub-segment of a sign..

Further Reading

• Al-Biruni, Kitāb al-Tafhīm
• Dorotheus of Sidon, Carmen Astrologicum
• Austin Coppock, 36 Faces: The History, Astrology, and Magic of the Decans