Decan

Astronomical ceiling of Senemut Tomb showing various decans, as well as the personified representations of stars and constellations

Decans first appeared in the 10th Dynasty (2100 BCE) on coffin lids.[2] 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[3] decans (36 × 10 = 360 days), plus five added days to compose the 365 days of a solar based year. Decans measure sidereal time and the solar year is six 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).[4]

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.[5]

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;[6] 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.[7]

Decans are named in various Greco-Egyptian sources, many Hermetic writings, the Testament of Solomon,[8] and the writings of Aristobulus of Paneas.[9] Julius Firmicus Maternus, Cosmas of Maiuma, Joseph Justus Scaliger, and Athanasius Kircher.[8]

Images of the decans are described in Hermetic writings, by the Indian astrologer Varāhamihira, in the Picatrix, and in Japanese writings.[10] Varāhamihira's images of the decans was influenced by Greco-Egyptian, if not Hermetic, depictions of the decans by way of the Yavanajataka.[11] Their role in Japanese astrology may have derived from an earlier Chinese[12] or Indian form[13] possibly from adding the twelve animals of the Chinese zodiac to a list of twenty-four hour stars.[12] They were most common between the Kamakura and Edo periods.[13]

The first original Decan position due to the precession in ancient times started at 0° of Cancer when the heliacal rising of Sirius (Egyptian Sepdet; Greco-Egyptian: Sothis) on sunset like Jewish and Islamic calendars marking the Egyptian New Year and now the 1st decan falls on 0° of Leo at July 20 in the Julian calendar, that is July 22/23 on the Gregorian calendar.

There were two main versions of rulership given to the decans in the ancient world: Chaldean rulership and rulership by Triplicity.

* 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.

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).[26]

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.[27]

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
• Decan (astrology)
• Drekkana
• Line of advantage
• Nakshatra
• Palazzo Schifanoia, Ferrara

• Symons, Sarah (2014). "Egyptian "Star Clocks"". In Ruggles, Clive L.N. (ed.). 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. doi:10.1086/370901. JSTOR 542436. S2CID 222443741. 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.

• Symons, S.L., Cockcroft, R., Bettencourt, J. and Koykka, C., 2013. Ancient Egyptian Astronomy. [Online database] Available at: <http://aea.physics.mcmaster.ca/>.
• Egypt Astronomy/Astrology: The decans
• Decan meanings, from Paul Christian's The History and Practice of Magic.