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.)
Decans continued to be used throughout the Renaissance in astrology and in magic, but modern astrologers almost entirely ignore them.
Ancient Egyptian origins
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. The ancient Book of Nut, covers the subject of the decans.
There were 36 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.
According to Sarah Symons,
Although we know the names of the decans, and in some cases can translate the names (Hry-ib wiA means ‘in the centre of the boat’) the locations of the decanal stars and their relationships to modern star names and constellations are not known. This is due to many factors, but key problems are the uncertainty surrounding the observation methods used to develop and populate the diagonal star tables, and the criteria used to select decans (brightness, position, relationship with other stars, and so on).
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.
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; 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.
Table of Faces (or Decanates)
|Sign||First Decan ruler
(0 - 9.999 deg.)
|Second Decan ruler
(10 - 19.999 deg.)
|Third Decan ruler
(20 - 29.999 deg.)
* as used as an essential dignity in astrology.
Notice that rulerships follow a repeating pattern, the so-called "Chaldean" 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.
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.
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.
- Asterism (astronomy)
- Astronomical ceiling of Senemut Tomb
- Egyptian astronomy
- Line of advantage
- von Bomhard, Dr. A. S., The Egyptian Calendar a Work for Eternity, London 1999, page 51
- Sarah Symons, A Star’s Year: The Annual Cycle in the Ancient Egyptian Sky - University of Leicester
- Neugebauer, Otto (1983) . "The Egyptian "Decans"". Astronomy and History: Selected Essays. New York: Springer. pp. 205–209. doi:10.1007/978-1-4612-5559-8. ISBN 978-0-387-90844-1. Neugebauer, Otto (1969) . The Exact Sciences in Antiquity (2 ed.). Dover Publications. pp. 81–88. ISBN 978-0-486-22332-2.
- see for example, RUELLE, C. E., Hermès Trismégiste, Le livre sacré sur les décans. Texte, variantes et traduction française, Revue de philologie, de littérature et d'histoire anciennes, n.s.:32:4 (1908:oct.) p .247
- Julius Firmicus Maternus, Matheseos IV/22.
- William Lilly, Christian Astrology (London, 1647), pp. 104, 105.
- Monier Williams Sanskrit Dictionary
- Pingree, David (1963). "The Indian Iconography of the Decans and Horas". Journal of the Warburg and Courtauld Institutes 26 (3/4): 223–254. doi:10.2307/750493 – via JSTOR. (registration required (. ))
- Symons, Sarah (2014). "Egyptian “Star Clocks”". In Ruggles, Clive L.N. Handbook of Archaeoastronomy and Ethnoastronomy. New York: Springer. pp. 1495–1500. ISBN 978-1-4614-6140-1.
- van der Waerden, B. L. (January 1949). "Babylonian Astronomy. II. The Thirty-Six Stars". Journal of Near Eastern Studies 8 (1): 6–26 – via JSTOR. (registration required (. "The property of the Chaldean Decans that one of them rose every ten days made them fit to be assimilated to the Egyptian decans. This assimilation was performed in the decan lists of Hellenistic astrology." ))
- Egypt Astronomy/Astrology: The decans
- Decan meanings, from Paul Christian's The History and Practice of Magic.
- Decan meanings, from Three Books of Occult Philosophy by Henry Cornelius Agrippa and Astrolabium Planum by Johannes Engel.