In astrology, a decan is the subdivision of a sign. In order to give fuller interpretation to the zodiac signs, ancient astrologers subdivided each sign into periods of approximately ten days. These divisions are known as the "decans" or "decanates" and cover modifications of individual traits, attributed to minor planetary influences, which temper or blend with the ruling influence of the period. The ten-day spans are somewhat arbitrary in order to allow for the five (and sometimes six) extra days in the year beyond the 360 days required for the thirty-six decans.
Each sign is allocated a triplicity, consisting of three of the four classical elements air, water, earth or fire, and is therefore subdivided into three equal parts of 10 degrees each; these parts are referred to as decans or decanates.
Each decan of a sign is assigned rulership by the planet ruling the sign and secondary rulership by the planet ruling the decan.
The decans and their rulers are assigned as follows as based on the description by the Alexandrian Egyptian astrologer, Ptolemy, from whose Tetrabiblos are derived most of the principles of western astrology:
| Sign | Domicile ruler | Exaltation ruler | 0–10 degrees | 10–20 degrees | 20–30 degrees | |
|---|---|---|---|---|---|---|
| Aries | Mars | Sun | Mars | Sun | Venus | |
| Taurus | Venus | Moon | Mercury | Moon | Saturn | |
| Gemini | Mercury | N/A | Jupiter | Mars | Sun | |
| Cancer | Moon | Jupiter | Venus | Mercury | Moon | |
| Leo | Sun | N/A | Saturn | Jupiter | Mars | |
| Virgo | Mercury | Mercury | Sun | Venus | Mercury | |
| Libra | Venus | Saturn | Moon | Saturn | Jupiter | |
| Scorpio | Mars | N/A | Mars | Sun | Venus | |
| Sagittarius | Jupiter | N/A | Mercury | Moon | Saturn | |
| Capricorn | Saturn | Mars | Jupiter | Mars | Sun | |
| Aquarius | Saturn | N/A | Venus | Mercury | Moon | |
| Pisces | Jupiter | Venus | Saturn | Jupiter | Mars |
| Sign | First Decan ruler(0–9.999 deg.) | Second Decan ruler(10–19.999 deg.) | Third Decan ruler(20–29.999 deg.) |
|---|---|---|---|
| Aries | Mars | Sun | Venus |
| Taurus | Mercury | Moon | Saturn |
| Gemini | Jupiter | Mars | Sun |
| Cancer | Venus | Mercury | Moon |
| Leo | Saturn | Jupiter | Mars |
| Virgo | Sun | Venus | Mercury |
| Libra | Moon | Saturn | Jupiter |
| Scorpio | Mars | Sun | Venus |
| Sagittarius | Mercury | Moon | Saturn |
| Capricorn | Jupiter | Mars | Sun |
| Aquarius | Venus | Mercury | Moon |
| Pisces | Saturn | Jupiter | Mars |
Notice that rulerships follow a repeating pattern, the Chaldaean order of the planets: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, based on a geocentric cosmological model.
Modern astrology updates the rulerships. Decans or "faces" are the least important of the essential dignities, representing about one-fifteenth of a planet's overall strength in medieval astrology.
The decans and their rulers are assigned as follows as based on the concepts of modern Western astrology :
| Sign | 1st decan ruler | 2nd decan ruler | 3rd decan ruler | |
|---|---|---|---|---|
| Aries | Mars | Sun | Jupiter | |
| Taurus | Venus | Mercury | Saturn | |
| Gemini | Mercury | Venus | Uranus | |
| Cancer | Moon | Pluto | Neptune | |
| Leo | Sun | Jupiter | Mars | |
| Virgo | Mercury | Saturn | Venus | |
| Libra | Venus | Uranus | Mercury | |
| Scorpio | Pluto | Neptune | Moon | |
| Sagittarius | Jupiter | Mars | Sun | |
| Capricorn | Saturn | Venus | Mercury | |
| Aquarius | Uranus | Mercury | Venus | |
| Pisces | Neptune | Moon | Pluto |