Decan (astrology)

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.^{[1][better source needed]}

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:^[2]

* as used as an essential dignity in astrology.

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.^{[4][better source needed]}

The decans and their rulers are assigned as follows as based on the concepts of modern Western astrology :

• Decans in the Chinese zodiac

• Anon (n.d.). "Scorpio the Scorpion". Novareinna.com. Archived from the original on 2010-01-03. Retrieved 2009-07-23.
• Jones, Cornelia (2011). "Decans in Astrology - Planetary Rulers". Horoscopes Within. Retrieved 2021-06-30.
• Lilly, William (1999a) [1647]. Houlding, D. (ed.). Christian Astrology, Book 1: An Introduction to Astrology; Book 2: The Resolution of All Manner of Questions. London: Ascella Publications.
• Ptolemy (1822). Ptolemy's Tetrabiblos or Quadripartite: Four Books of the Influence of the Stars. Translated by J. Ashmand. London: Davis and Dickson.

• Beros, Matt (September 21, 2017). "John Dee's Astronomicon and the Thirty-Six Decans". Archaeologyofreading.org. Retrieved 2021-07-05.
• Lilly, William (1999b) [1647]. Houlding, D. (ed.). Christian Astrology, Book 3: An Easie And Plaine Method Teaching How to Judge upon Nativities. London: Ascella Publications.
• Ptolemy (1940). Tetrabiblos. Translated by Frank Egleston Robbins. Cambridge, MA: Loeb. Retrieved 2021-07-05.

• Image for the Third Face of Aries, an example of a decan talisman using traditional astrology.