decans
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thumb|upright=1.35|'Diagonal star table' from the late Eleventh Dynasty of Egypt|Eleventh Dynasty coffin lid; found at [[Asyut, Egypt. Roemer- und Pelizaeus-Museum Hildesheim]] The decans (; ) are 36 groups of stars (small constellations) used in ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, both for theurgical and heliacal chronometrical 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"
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9 sectionsContents
- Ancient Egyptian origins
- Later developments
- Descriptions of the decans
- Ancient India
- See also
- References
- Works cited
- Further reading
- External links
thumb|upright=1.35|'Diagonal star table' from the late Eleventh Dynasty of Egypt|Eleventh Dynasty coffin lid; found at [[Asyut, Egypt. Roemer- und Pelizaeus-Museum Hildesheim]] The decans (; ) are 36 groups of stars (small constellations) used in ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, both for theurgical and heliacal chronometrical 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 nocturnal (sidereal clock) beginning by at least the Ninth or Tenth Dynasty of Egypt in the 21st century BC.
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 Greeks called them dekanói (; pl. of dekanós) or "tenths".