Aries — 1st Decan (0°–10°)
The first decan of Aries covers 0° to 10° of the sign. It's the one point where the two major decan-rulership systems coincide: under both Triplicity and the Chaldean system, this decan is ruled by Mars, Aries' own ruler.
Last reviewed: 2026-07-20
Historical fact
Decans originate in Ancient Egypt, long before the 12-sign zodiac existed: they were 36 small constellations ('decans') that rose successively on the horizon at roughly ten-day intervals, serving as a nighttime star clock documented on coffin lids and astronomical tables dating back to around 2100 BCE. Only in the Hellenistic period, when Greek astronomers merged this Egyptian system with the Babylonian 12-sign zodiac, did each sign come to be divided into 3 decans of 10°, each assigned a planetary ruler — giving rise to the two rulership systems documented today.
Interpretation — Triplicity System (Modern Astrology)
In the Triplicity system, each sign's decans are ruled, in order, by the sign itself and the other two signs sharing its element. For Aries (Fire), this means: 1st decan ruled by Mars (Aries itself), 2nd by the Sun (Leo), and 3rd by Jupiter (Sagittarius). This first decan is read as the purest, least diluted expression of Aries energy — initiative, impulse and direct action, unmixed with any other sign.
Interpretation — Chaldean System (Hellenistic Tradition)
In the older Chaldean system, rulers follow one continuous cycle of the 7 classical planets across all 36 decans, without resetting at each sign — and that cycle also begins with Mars at Aries' first decan. This convergence is a structural coincidence between the two systems, not the rule: starting from Aries' third decan, the two systems already diverge.
This section presents one tradition's interpretation, not a verifiable fact.
Frequently asked questions
- Are decans older than the 12-sign zodiac?
- Yes. The documented Egyptian decans exist from around 2100 BCE, used as a star clock — long before the Babylonian 12-sign zodiac was systematized. The merger of the two systems only occurred in the Hellenistic period.
- Why does Aries' 1st decan have the same ruler in both systems?
- Because both systems assign the sign's own ruler to the first decan — in Aries' case, Mars. This coincidence doesn't repeat across every decan: starting from Aries' 3rd decan, the Triplicity and Chaldean systems already indicate different rulers.
Sources
- Nicholas Campion. A History of Western Astrology (2009).
- Claudius Ptolemy. Tetrabiblos (c. 150).
- Vettius Valens. Anthology (Hellenistic astrology) (c. 175).
- Otto Neugebauer, Richard A. Parker. Egyptian Astronomical Texts (1960).