page_title: Decan
section_title: Ancient Egyptian origins
hierarchical_section_title: Decan / Ancient Egyptian origins
caption_attribution_description: English: The double wall mural of the tomb of Senenmut.
context_page_description: The decans are 36 groups of stars used in the ancient Egyptian astronomy. They rose consecutively on the horizon throughout each earth rotation. The rising of each decan marked the beginning of a new decanal "hour" 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.
  Because a new decan also appears heliacally every ten days, the ancient Greeks called them dekanoi or "tenths".
  Decans continued to be used throughout the Renaissance in astrology and in magic, but modern astrologers almost entirely ignore them.
context_section_description: 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 (Hry-ib wiA 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 many factors, but key problems are the uncertainty surrounding the observation methods used to develop and populate the diagonal star tables, and the criteria used to select decans (brightness, position, relationship with other stars, and so on).
caption: Astronomical ceiling of Senemut Tomb showing various decans, as well as the personified representations of stars and constellations
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