Astrology Decans: Modern Triplicity vs Chaldean Rulers Compared

Every 30° zodiac sign is divided into three 10° decans (36 decans across the 360° ecliptic), but two major traditions assign different planetary rulers to them. Comparing the modern element-triplicity system with the traditional Hellenistic Chaldean faces across all 36 decans shows that the two systems assign the same planetary ruler in only 6 of 36 decans (16.67%).

Look up your Sun, Moon and rising decan in both systems at once with the decan calculator.

How the two decan rulership systems work

• Modern triplicity system: The first decan (0°–10°) is sub-ruled by the sign itself and its modern planetary ruler; the second decan (10°–20°) by the next sign of the same element (4 signs forward); and the third decan (20°–30°) by the third sign of that element (8 signs forward). It includes Uranus, Neptune and Pluto for Aquarius, Pisces and Scorpio.
• Traditional Chaldean faces: Beginning with Mars for the first decan of Aries (0°–10° Aries), the seven classical bodies repeat continuously in descending orbital order—Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon—through all 36 decans. Because 36 = 5 × 7 + 1, the 36th decan (20°–30° Pisces) is also ruled by Mars.

Complete 36-decan ruler comparison table

The six decans where both traditions name the same planet are Aries 1 (Mars), Aries 2 (the Sun), Taurus 3 (Saturn), Leo 2 (Jupiter), Leo 3 (Mars) and Aquarius 2 (Mercury).

Why fixed calendar dates for Sun decans can be off by a day

Many astrology sites define decans by fixed 10-day birthday slices (such as “March 21–30, March 31–April 9, April 10–19”). Astronomically, a decan is defined by 10° of ecliptic longitude (0°–10°, 10°–20°, 20°–30°), not a count of calendar days. Because of the leap-year cycle and Earth’s elliptical orbit (the Sun takes ~31.4 days to cross Cancer in northern summer, but only ~29.4 days to cross Capricorn in northern winter), exact decan entry times shift each year:

Method and limits

In our decan calculator, a planet at longitude λ belongs to decan floor((λ mod 30) / 10) + 1, using unrounded tropical longitudes. Boundary crossings are solved by Newton steps on solar longitude. Both rulership systems are historical symbolic conventions rather than empirical classifications.

Download the research and comparison JSON · Reproduce these tables (TypeScript source). Output is licensed under CC BY 4.0; credit Natal Chart Lab and link to this article.

Prepared with AI assistance and checked against calculation code and saved data. Astronomical calculations and dataset statistics do not establish astrological claims about personality, relationships or future events.