Decans: the 36 subdivisions behind your sign
Thirty-six faces, two rulership schemes, and an origin older than the zodiac.
2 min read · Published 29 August 2026
The answer
Each of the twelve signs divides into three ten-degree segments, giving 36 decans. Two rulership schemes compete: the triplicity system assigns each decan to a sign of the same element in order, while the older Chaldean system runs the seven visible planets through all 36 faces in sequence. The idea descends from Egyptian star groups used to tell the time at night.
The two schemes
| Triplicity | Chaldean | |
|---|---|---|
| Rule | Your element's three signs, in order from your own | Seven planets in descending apparent speed, cycling through all 36 |
| Aries I / II / III | Aries, Leo, Sagittarius | Mars, Sun, Venus |
| Taurus I / II / III | Taurus, Virgo, Capricorn | Mercury, Moon, Saturn |
| Used by | Most modern astrology | Traditional practice and tarot |
| Repeats? | Every sign, cleanly | No. 36 does not divide by 7, so it never closes neatly |
Because 36 faces cycle through 7 planets, the Chaldean sequence never closes neatly: Pisces III lands on Mars, where Aries I began, purely by where the count falls.
Where they come from
The decans predate the zodiac. Egyptian astronomers tracked 36 star groups whose risings marked the hours of the night, each rising about ten days apart. Thirty-six tens gives 360 days, the Egyptian civil year and, in the end, the 360-degree circle. Hellenistic astrology mapped them onto the zodiac; the rulerships came later.
Chaldean decans for me, every time. The triplicity scheme is tidy, but the older one is the one with a history.
What decans do interpretively
They give a structural reason why a late Aries reads differently from an early one, the defensible version of what the cusp idea fumbles. Aries II borrows from Leo, Aries III from Sagittarius, and all three stay Aries.
Which decan your Sun falls in
0–9° is the first decan, 10–19° the second, 20–29° the third, for every planet. AsterNym lists all three decans with both schemes on every sign page and computes yours on your natal chart.
What is a decan in astrology?
A ten-degree subdivision of a sign. Each sign has three, 36 in all, and yours is set by your degree: 0–9°, 10–19° or 20–29°.
What are the two decan rulership systems?
Triplicity assigns each decan to one of your element's signs in order. Chaldean runs the seven visible planets in order of apparent speed through all 36 faces, starting with Mars at Aries I.
Where did decans originate?
In ancient Egypt, as 36 star groups whose risings marked the hours of the night. Hellenistic astrology later mapped them onto the zodiac.
Mira Hale