Decan

A decan is one of the three 10-degree divisions of a zodiac sign, making thirty-six decans in all. Inherited from ancient Egypt, where they designated groups of stars used to measure the hours of the night, they were incorporated into Hellenistic astrology. Each decan is given a planetary sub-ruler, according to systems that vary: the Chaldean one, or the more modern system based on signs of the same element. Decans are used to refine the portrait of a sign.