The Decans (Egyptian bakiu) are 36 groups of stars (small constellations) which rise consecutively on the horizon throughout each earth rotation. The rising of each decan marked the beginning of a new decanal "hour" (Greek hōra) of the night for the ancient Egyptians, and they were used as a sidereal star clock beginning by at least the 9th or 10th Dynasty (ca 2100 BCE.)
Because a new decan also appears heliacally every ten days (that is, every ten days, a new decanic star group reappears in the eastern sky at dawn right before the Sun rises, after a period of being obscured by the Sun's light), the ancient Greeks called them dekanoi (pl. of dekanos) or "tenths" (and when the concept of decans reached northern India, they were called drekkana in Sanskrit.) These predictable heliacal re-appearances by the decans were eventually used by the Egyptians to mark the divisions of their annual solar calendar. Thus the heliacal rising of Sirius marked the annual flooding of the Nile.
Eventually this system led to a system of 12 daytime hours and 12 nighttime hours, varying in length according to the season. Later, a system of 24 "equinoctial" hours was used.[1]
After astrology was introduced to Egypt, various systems attributing astrological significance to decans arose. Decans were connected, for example, with various diseases and with the timing for the engraving of talismans for curing them;[2] with decanic "faces" (or "phases"), a system where three decans are assigned to each zodiacal sign, each covering 10° of the zodiac, and each ruled by a planetary ruler (see below); and correlated with astrological signs.[3]
Decans continued to be used throughout the Renaissance in astrology and in magic, but modern astrologers almost entirely ignore them.
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Decans first appeared in the 10th Dynasty (2100 BCE) on coffin lids. The sequence of these star patterns began with Sothis/Sirius, and each decan contained a set of stars and corresponding divinities. As measures of time, the rising and setting of decans marked 'hours' and groups of 10 days which comprised an Egyptian year.
There were 36 [4] decans (36 X 10 = 360 days). plus 5 added days to compose the 365 days of a solar based year. Decans measure sidereal time and the solar year is 6 hours longer; the Sothic and solar years in the Egyptian calendar realign every 1460 years. Decans represented on coffins from later dynasties (such as King Seti I) compared with earlier decan images demonstrate the Sothic-solar shift.
| 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 so-called "Chaldaean" order of the planets: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. This planetary order, in which the Sun stands at the center of the continuum, with the planets between the Sun and the Earth on one side and the outer planets on the other side, reflected the perception of the speed of each planet's motion as seen from the Earth.
Decans or "faces" are the least important of the essential dignities, representing about one-fifteenth of a planet's overall strength in medieval astrology.
In India, the division of the zodiac into 36 ten degree portions is called either the drekkana (drekkāṇa),the dreshkana (dreṣkāṇa), or the drikana (dṛkāṇa).[6]
The iconography and use of the drekkana’s is mention earliest by Sphujidhvaja in Yavanajataka (269-70 CE), and given detailed treatment by Varahamihira in his Brihat-Samhita (550 CE). Modern scholars believe the decans were imported into India through the Greeks, who learned about them from the Egyptians.[7]
There are multiple types of drekkana in use in Indian astrology. The parivritti drekkana goes in order of the signs; the first decan is Aries, the second is Taurus, the third is Gemini, the fourth is Cancer, etc. Then there is the trinal calculation which utilizes the elemental trines to each sign; In Aries there is Aries, Leo, and Sagittarius, while in Taurus there is Taurus, Virgo, and Capricorn. There are in total four variations of drekkana calculations. Indian astrologers will calculate these signs (varga) and create a new chart based upon the sign placement for predictive purposes.