Decans

The decans (/ˈdɛkənz/; Egyptian bakiu) are 36 groups of stars (small constellations) which rise consecutively on the horizon throughout each earth rotation. The rising of each decan marked the beginning of a new decanal "hour" (Greek hōra) of the night for the ancient Egyptians, and they were used as a sidereal star clock beginning by at least the 9th or 10th Dynasty (c. 2100 BCE).

Because a new decan also appears heliacally every ten days (that is, every ten days, a new decanic star group reappears in the eastern sky at dawn right before the Sun rises, after a period of being obscured by the Sun's light), the ancient Greeks called them dekanoi (δεκανοί; pl. of δεκανός dekanos) or "tenths" (and when the concept of decans reached northern India, they were called drekkana in Sanskrit.)

Decans continued to be used throughout the Renaissance in astrology and in magic, but modern astrologers almost entirely ignore them.

Ancient Egyptian origins

Decans first appeared in the 10th Dynasty (2100 BCE) on coffin lids. 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.