The decans (; Egyptian bꜣktw or baktiu, "[those] connected with work") are 36 groups of stars (small constellations) used in the ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, both for theurgical and heliacal horological purposes. The decans each appeared, geocentrically, to rise consecutively on the horizon throughout each daily 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". Decans gave way to a lunar division of 27 or 28 lunar stations, also known as manzil, lunar mansions or nakshatras and thence to a zodiac of 12 signs, based on an anthropomorphic pattern of constellations, and their use can be seen in the Dendera zodiac dated to circa 50 BCE. 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. There were 36 decans (36 × 10 = 360 days), plus five added days to compose the 365 days of a solar based year. Decans measure sidereal time and the solar year is six hours longer; the Sothic and solar years in the Egyptian calendar realign every 1460 years. Decans represented on coffins from later dynasties (such as King Seti I) compared with earlier decan images demonstrate the Sothic-solar shift. According to Sarah Symons: Although we know the names of the decans, and in some cases can translate the names (ḥry-ỉb wỉꜣ means 'in the centre of the boat') the locations of the decanal stars and their relationships to modern star names and constellations are not known. This is due to