By Dorian Grimault, astrologer · Published · updated
The decans divide each sign into three equal slices of 10°. This is one of the oldest divisions of the zodiac, coming from Egypt, and it has known several systems of attribution. AstralPro uses one, the decan by element with a modern sub-ruler, and clearly distinguishes it from the Chaldean faces of the tradition. Here is how it is calculated and how it is read, with the example of Camille Morel, AstralPro's sample chart (fictional data).
The Egyptians followed thirty-six groups of stars, the decans, which rose one after another on the horizon and were used to count the hours of the night. When Hellenistic astrology brought them into the zodiac, each sign received three of them, 10° each. That left the question of who rules each slice, and two main answers have coexisted since Antiquity.
The first is that of the faces, called Chaldean: you follow the order of the seven planets (Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter) starting from the first decan of Aries, and repeat it across the thirty-six decans. The second, found in Indian astrology and then taken up by modern astrology, links each decan to a sign of the same element.
AstralPro calculates the decan by element. The first decan of a sign refers to the sign itself, the second to the next sign of the same element, the third to the last one. The sub-ruler is the modern ruler of that associated sign. For Scorpio, a Water sign:
For Aries, a Fire sign, the sequence gives Mars, then the Sun (Leo), then Jupiter (Sagittarius). Because the rulerships are modern, Uranus, Neptune and Pluto can be sub-rulers: this is the most visible difference from the traditional tables, which know only seven.
The faces have not disappeared from the engine, but they serve only one purpose: the scale that adds up the dignities of a degree to weigh the Ascendant, where the face is worth 1 point. They never appear in the decan paragraph of a planet block, and the word “face” does not figure in the prose.
The two systems do not answer the same question and often diverge: out of the thirty-six decans, thirty give a different ruler. Aries offers a simple example, Mars, Sun, Jupiter by element, Mars, Sun, Venus by faces. For Scorpio, the faces give Mars, Sun and Venus, where AstralPro reads Pluto, Neptune and the Moon.
When you read “decan” in an AstralPro report, it therefore always means the decan by element.
In Chapter I, each planet's block devotes a paragraph to the decan and term of the degree occupied. The decan is written there with its number, its sub-ruler and the associated sign, in the form “In the 2nd decan (sub-ruler Mercury, Gemini) in Venus terms”, followed by a brief interpretation. The sub-ruler brings a secondary coloring: the planet keeps the nature of its sign and borrows part of that of the associated sign.
The glossary at the end of the report defines the Decan as the division of a sign into three 10° slices, each with its own coloring. The decan changes neither the dignity nor the house rulership: it nuances, it does not decide.
One consequence of the calculation is worth knowing. In the 1st decan, the associated sign is the sign itself, and the sub-ruler merges with the ruler of the sign: the decan then doubles the color of the sign instead of shifting it. In the 2nd and 3rd decans, it opens it toward another sign of the same element, so that the nuance stays within the same family. A Scorpio decan refers only to Scorpio, Pisces or Cancer, never to a Fire, Earth or Air sign.
Camille's Sun is at 21°55' Scorpio: past 20°, it occupies the 3rd decan, the one that refers to Cancer, and its sub-ruler is the Moon. Its technical paragraph can therefore open with “In the 3rd decan (sub-ruler Moon, Cancer) in Jupiter terms”. Scorpio gives intensity and restraint, the associated Cancer a tint of protection and attachment to one's own.
Camille shares this decan with two other bodies: her Moon (27°09') and her Pluto (25°17') are also in the 3rd decan, sub-ruler Moon. Mercury (6°37'), Venus (6°30') and Jupiter (0°50') are in the 1st decan, sub-ruler Pluto: for them, the decan doubles the Scorpio tone. Outside Scorpio, Mars and the North Node, in the 1st decan of Sagittarius, have Jupiter; Saturn, in the 3rd decan of Aquarius, has Venus (Libra); Uranus and Neptune, in the 2nd decan of Capricorn, have Venus (Taurus); Lilith, in the 2nd decan of Aries, has the Sun (Leo); Chiron, in the 1st decan of Virgo, has Mercury.
By the Chaldean faces, Camille's Sun would have Venus as decan ruler. It is the Moon that her report uses.
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