Decans are used by astrologers in birth charts to take a deeper dive into your astrological personality in order to give a more individualized reading, therefore, if you want to know what makes you–you–explore your decans!
Decans go beyond rising signs in complexity to assign planetary sub-rulers to the sign, which provide additional layers of symbolism that can be applied to your birthdate within your sign, Well + Good explains.
The word decan comes from the Latin word decanus, which literally means, chief of ten, according to Merriam-Webster.
In astrology, decan refers to “any of the three divisions of 10 degrees in each sign of the zodiac.” It can also refer to “the ruler of such a division.”
The zodiac spans 360 degrees, with each of the 12 signs occupying 30 degrees.
Ancient astrologers subdivided the 30 degrees that each zodiac sign spans into three periods of approximately ten days, thus, this results in three sections of 10 degrees referred to as decans. Therefore, in the 360 degrees that make up the zodiac, decans makeup 36 segments of the ecliptic, consisting of 10 degrees each.
There are actually two different systems of decans that astrologers use.
The first is called a triplicity system. This system assigns the same element of the sign to each decan.
For example, if we are working with the element of fire, we have the fire signs of Aries, Leo and Sagittarius. Therefore, the first 10 degrees would belong to the sign itself, so we have Aries/Aries. The next 10 degrees would belong to the next sign within the same triplicity or element, so we would get Aries/Leo. The final 10 degrees would give us Aries/Sagittarius.
Aries/Aries, Aries/Leo, Aries/Sagittarius
Leo/Leo, Leo/Sagittarius, Leo/Aries
Sagittarius/Sagittarius, Sagittarius/Aries, Sagittarius/Leo
Pisces/Pisces, Pisces/Cancer, Pisces/Scorpio
Cancer/Cancer, Cancer/Scorpio, Cancer/Pisces
Scorpio/Scorpio, Scorpio/Pisces, Scorpio/Cancer
Taurus/Taurus, Taurus/Virgo, Taurus/Capricorn
Virgo/Virgo, Virgo/Capricorn, Virgo/Taurus
Capricorn/Capricorn, Capricorn/Taurus, Capricorn/Virgo
Aquarius/Aquarius, Aquarius/Gemini, Aquarius/Libra
Gemini/Gemini, Gemini/Libra, Gemini/Aquarius
Libra/Libra, Libra/Aquarius, Libra/Gemini
The second system of decans is called the Chaldean system. This system assigns one of the seven planets that are visible to the naked eye, (including the Sun and Moon) to each 10 degree decan. The Chaldean system is less popular in modern astrology with the triplicity system being more prominently used.
As already mentioned above, within the 360 degrees that make up the zodiac, decans consist of 10 degrees each, three per each of the 12 zodiac signs, comprising 36 segments of the ecliptic.
The Chaldean system assigns three planets to each zodiac sign, in the following order, for each 10 degree decan, according to The Astrology Dictionary.
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
Therefore, the first 0-10 degrees in Aries would belong to Mars, the second 10-20 degrees of Aries would belong to the son, and the third 20-30 degrees of Aries would belong to Venus.
*Note: the above list is the assignment in Ancient Western Astrology. The modern system uses a different assignment of planets which can be found on Wikipedia.