 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".