Decans are one of the oldest astrological systems — 36 stars that ancient Egyptians used for timekeeping as far back as 2100 BCE. In Hellenistic astrology, each zodiac sign is divided into three decans of 10 degrees each, and each decan receives a secondary ruler according to the triplicity system (same element, next sign). the tropical degree of the Sun, Moon and Ascendant and determines which of the 36 faces of the zodiac rules each. The interpretation covers how the secondary ruler modifies the primary sign energy — what it adds, complicates or deepens in identity, emotions and first impression — and how the three decan rulers interweave into a unique archetypal portrait.

Where it comes from

The decans are the oldest surviving astronomical scheme of any kind. Egyptian coffin lids from around 2100 BCE carry tables of thirty-six stars whose successive risings divided the night, and those divisions gave Egypt the twenty-four-hour day. When Greek astrology absorbed Egyptian material the thirty-six decans were laid over the twelve signs, three to a sign, and each was given a planetary ruler — the arrangement the Hellenistic and later medieval sources use.

How the method works

Each sign of thirty degrees is cut into three faces of ten. The first decan of a sign is ruled by the sign's own ruler; the second and third pass to the rulers of the following signs of the same element. So Aries gives its first ten degrees to Mars, the next ten to the Sun as ruler of Leo, the last ten to Jupiter as ruler of Sagittarius. The secondary ruler does not replace the sign, it inflects it — a late Aries Sun is still Aries, but read through Jupiter rather than through Mars alone. Applied to the Sun, Moon and Ascendant it produces three inflections that have to be read together.

What a reading covers

A reading gives the decan of your Sun, Moon and Ascendant with its secondary ruler, what that ruler adds to or complicates in each, and how the three decan rulers combine into one portrait.

Common questions

Is this the same as the Egyptian star decans?: It descends from them but is not identical. The Egyptian scheme tracked actual star risings; the astrological one is a fixed ten-degree division of the zodiac with planetary rulers added in the Hellenistic period.
My Sun is at 10 degrees — which decan is that?: Degrees 0 through 9 are the first decan, 10 through 19 the second, 20 through 29 the third. A Sun at exactly 10°00' is in the second.
There are other decan rulership schemes — which one is used here?: The triplicity scheme, where the rulers follow the signs of the same element. The other common one is the Chaldean order, used mainly in magical and tarot contexts.