Decans: the 36 subdivisions behind your sign
Thirty-six faces, two competing rulership schemes, and an origin older than the zodiac itself.
3 min read · Published 29 August 2026
The short answer
Each of the twelve signs divides into three ten-degree segments, giving 36 decans across the zodiac. 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 concept descends from Egyptian star groups used for night-time timekeeping.
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 |
That last row is worth noticing. Because 36 faces cycle through 7 planets, the sequence does not return to its start — Pisces III lands on Mars, the same planet that begins Aries I, purely because of where the count falls. It is a genuine feature of the scheme rather than a design.
Where they come from
The decans predate the zodiac. Ancient Egyptian astronomers tracked 36 star groups whose successive risings marked the hours of the night — a timekeeping system, with each decan rising roughly ten days apart across the year.
Thirty-six decans of ten days each gives 360 days, which is the basis of the Egyptian civil year and, ultimately, of the 360-degree circle.
When Hellenistic astrology absorbed Egyptian material, the decans were mapped onto the zodiac as ten-degree divisions. The planetary rulerships came later, and the triplicity scheme later still.
So the structure is genuinely ancient and the meanings attached to it are not — the usual pattern, and the same one described in the history of tarot.
What decans do interpretively
They give a structural reason why a late Aries reads differently from an early one — which is the genuine version of what the cusp idea gestures at incorrectly.
Under triplicity, Aries I is Aries undiluted, Aries II borrows from Leo, and Aries III from Sagittarius. All three remain Aries; the flavour shifts.
This is more defensible than most degree-level techniques because it is a structural division with a rule, not an assignment of meaning to individual degrees. Compare degree symbolism, which has considerably thinner provenance.
Finding yours
It follows from your Sun's degree: 0–9° is the first decan, 10–19° the second, 20–29° the third. Every planet has a decan, not just the Sun.
AsterNym lists all three decans with both rulership schemes on every sign page, and computes your own Sun's decan on your natal chart.
We show both schemes rather than picking, because they genuinely disagree and neither has authority over the other.
Common questions
- What is a decan in astrology?
- A ten-degree subdivision of a zodiac sign. Each sign contains three, giving 36 across the zodiac. Your decan is determined by your degree within a sign: 0–9°, 10–19° or 20–29°.
- What are the two decan rulership systems?
- The triplicity scheme assigns each decan to one of your element's three signs in order, and is the modern default. The Chaldean scheme runs the seven visible planets in order of apparent speed through all 36 faces, starting with Mars at Aries I, and is used in traditional and tarot work.
- Where did decans originate?
- In ancient Egypt, as 36 star groups whose successive risings marked the hours of the night. Thirty-six ten-day periods gave a 360-day year, which is also the basis of the 360-degree circle. Hellenistic astrology later mapped them onto the zodiac.
- Why does tarot have 36 numbered minor cards?
- Because the Golden Dawn mapped the cards from Two to Ten across four suits — 36 in total — onto the 36 decans, using the Chaldean planetary rulerships. That is the origin of tarot correspondence tables assigning each card a sign and planet.