Decan
every reader
What the tradition reads
A decan, sometimes called a face, is one of three ten degree divisions within a thirty degree zodiac sign, so that the whole 360 degree circle of the zodiac is cut into thirty six decans in total. Wikipedia's account of the decan's later history notes that once the Egyptian scheme entered horoscopic astrology, various systems arose linking the decans to the classical planets, the five wandering stars together with the Sun and Moon, without settling on one single method every later school has kept. Wikipedia's page on the decan in astrology gives, for both the table it attributes to Ptolemy and the later table it calls the traditional Chaldean rulerships, one unbroken cycle of the seven planets in the Chaldean order, Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, run continuously across all thirty six decans; the same page shows modern astrology updating the rulers sign by sign to include Uranus, Neptune and Pluto instead. Whichever scheme fixes the ruler, the tradition reads the decan as a finer degree of texture than the sign alone provides: a planet or the Sun placed in a sign's third decan, for instance, is read as tempered by whatever rules that decan, in addition to and sometimes in some tension with the sign's own basic nature.
The story
Decans originate far earlier than the horoscopic astrology that later adopted them, in the astronomy of ancient Egypt, where thirty six groups of stars, rising roughly ten days apart across the year, were used from at least the First Intermediate Period to mark the passing hours of the night and to divide the year into thirty six ten day weeks. Because thirty six decans of ten days each give three hundred and sixty days, with five further days added to complete the solar year, the decan system sat at the root of the Egyptian civil calendar as much as of any astrological practice. When Egyptian and Greek astronomy merged in Hellenistic Alexandria, astrologers including Ptolemy and Vettius Valens carried the decans over into the emerging horoscopic system, re-mapping the old star groups onto the twelve tropical signs at ten degrees apiece and assigning each a planetary ruler and a set of symbolic images, a transformation visible in the surviving Dendera zodiac of around 50 BCE, which shows the decans alongside the newer sign figures. From Alexandria the divisions travelled on to India, where Sanskrit texts describe an equivalent ten degree division called the drekkana, first attested in Sphujidhvaja's Yavanajataka of 269 to 270 CE and treated at length by Varahamihira not long after.
Written from
Corpus key glossary:decan.