Decan, in full
The division is Egyptian and older than the zodiac itself. Egyptian astronomers tracked 36 star groups whose successive risings just before dawn marked the year off in ten-day stretches, matching a civil calendar of twelve 30-day months plus five extra days. Diagonal star tables painted inside Middle Kingdom coffin lids are the earliest surviving records of the scheme. Hellenistic astrologers later folded the 36 decans onto the twelve signs, three to a sign.
Two assignment schemes are in wide circulation and they do not agree. The triplicity version, common in modern popular astrology, gives a sign's first decan to that sign's own ruler, the second to the ruler of the next sign of the same element, and the third to the ruler of the last one. The older version, the faces, runs the seven classical planets in Chaldean order, from Saturn down through Jupiter, Mars, the Sun, Venus, Mercury and the Moon, straight through all 36 slots without restarting at each sign.
Under either scheme a decan is treated as a shading on the sign rather than a sign of its own. Someone at 5 degrees of Leo and someone at 25 degrees of Leo are both Leos. Because the two tables hand the same ten degrees to different planets, any specific decan meaning depends on which system the writer used, and writers rarely say which.
- Astrology
- Decanate, Face
A worked example
Aries covers 0 to 30 degrees, so its decans are 0-10, 10-20 and 20-30. Under the triplicity scheme they go to Mars, the Sun and Jupiter, the rulers of the three fire signs in wheel order. Under the older faces scheme the first two match, Mars then the Sun, but the third falls to Venus, because that sequence just keeps counting down the Chaldean order rather than restarting.
Sources
The decans are 36 Egyptian star groups that divided the year into ten-day stretches, first recorded on Middle Kingdom coffin lids as diagonal star tables and later assigned three to each zodiac sign by Hellenistic astrologers.
Wikipedia — Decan
See also
Where it comes up on this site
Decan: frequently asked
Take the Sun's degree within the sign from a chart and divide by ten. Under 10 degrees is the first decan, 10 to just under 20 the second, the rest the third. Published date ranges approximate that. The degree figure settles it.
No. All three decans of Leo are Leo.
From Egyptian timekeeping. Thirty-six star groups rose in sequence through the night, each announcing a ten-day stretch, which suited a civil calendar of twelve 30-day months plus five days. Hellenistic astrologers inherited the 36 divisions and reassigned them three to a zodiac sign.