Each sign of the zodiac occupies an arc of 30° (the 360° of a circle divided by 12 signs equals 30° per sign). Every sign is subdivided into 3 decans or decanates of 10°, each of which is associated with three signs of the same element. Thus, for example, Scorpio, a water sign, is subdivided into a Scorpio decan (from 0° Scorpio to 10° Scorpio), a Pisces decan (from 10° Scorpio to 20° Scorpio), and a Cancer decan (from 20° Scorpio to 30° Scorpio). The uniform pattern of decans is for the first 10° of every sign to be associated with the same sign. In other words, the first decan of Aries is the Aries decan, the first decan of Taurus is the Taurus decan, and so forth.
The second decan is associated with the next sign of the same element in the natural order of the zodiac. Thus, for instance, the second decan of Aries is the Leo
decan (the next fire sign), the second decan of Taurus is the Virgo decan (the next earth sign), etc.
The third decan is associated with the remaining sign of the same element. To continue with the same examples, the third decan of Aries is the Sagittarius decan, and the third decan of Taurus is the Capricorn decan.
For signs placed later in the zodiac, one continues around the zodiac, past the Pisces-Aries cusp (the boundary between the last and first signs), to pick up the next signs of the same element past this cusp. For example, for the sign Aquarius, which is the last air sign in the zodiac, the first decan is the Aquarius decan, the second decan is the Gemini decan (Gemini is the next air sign after one crosses the Pisces-Aries divide), and the third decan is Libra (the next air sign in succession). These relationships are clearer in outline form:
Sagittarius decan, 20°-30°
Taurus-Taurus decan, 0°-10° Virgo decan, 10°-20° Capricorn decan, 20°-30° Gemini-Gemini decan, 0°-10° Libra decan, 10°-20° Aquarius decan, 20°-30° Cancer-Cancer decan, 0°-10° Scorpio decan, 10°-20 ° Pisces decan, 20°-30° Leo-Leo decan, 0°-10° Sagittarius decan, 10°-20° Aries decan, 20°-30° Virgo-Virgo decan, 0°-10° Capricorn decan, 10°-20° Taurus decan, 20°-30° Libra-Libra decan, 0°-10° Aquarius decan, 10°-20° Gemini decan, 20°-30° Scorpio-Scorpio decan, 0°-10° Pisces decan, 10°-20° Cancer decan, 20°-30° Sagittarius-Sagittarius decan, 0°-10° Aries decan, 10°-20° Leo decan, 20°-30° Capricorn-Capricorn decan, 0°-10° Taurus decan, 10°-20° Virgo decan, 20°-30° Aquarius-Aquarius decan, 0°-10° Gemini decan, 10°-20° Libra decan, 20°-30° Pisces-Pisces decan, 0°-10° Cancer decan, 10°-20° Scorpio decan, 20°-30 °
The decans indicate a subsidiary influence. For example, someone born when the Sun was in the middle of Scorpio will be slightly influenced by the sign Pisces (because the Sun was in the Pisces decan of Scorpio); although the person will still be Scorpio, the normal intensity of this sign will be somewhat moderated by Pisces. This modification is relatively minor and is usually ignored, unless one is examining the subtleties of a particular chart. When decans are used at all, the focus is almost always the sun sign. For example, the decans of the planets and the other points in a natal chart, with the possible exception of the Moon and the ascendant (rising sign), are usually ignored.
Bach, Eleanor. Astrology from A to Z: An Illustrated Source Book. New York: Philosophical Library, 1990.
Lee, Dal. Dictionary of Astrology. New York: Paperback Library, 1969.
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