Decanal Hour Calculator

The 36 decans (Greek dekanoi, Egyptian baktiu) are a series of small constellations or asterisms that ancient Egyptian astronomers used to divide the celestial belt south of the ecliptic into thirty-six equal parts of ten degrees each. Each decan rose heliacally about ten days apart over the course of the year, providing a stellar clock that tracked both the solar year and the night hours. Twelve decans were visible above the horizon during any given night, and their successive risings or transits defined the twelve hours of the night that became standard in the Late Period and were inherited by the Greco-Roman world.

The astronomical ceiling of Senenmut's tomb (Theban Tomb 353), painted around 1473 BCE during the reign of Hatshepsut, is the earliest preserved depiction of the full Egyptian astronomical system, including the diagonal star table of the decans, the planets and the northern circumpolar constellations. Together with the Asyut coffin lids of the First Intermediate Period (c. 2150 BCE) it provides the foundational textual evidence for the use of decans as an hour-marking device. The Senenmut ceiling shows the decans listed in their sequence of heliacal rising, anchoring the 365-day civil year to the stellar cycle.

The Asyut diagonal star clocks of the Middle Kingdom record decanal hours by the heliacal rising of each decan above the eastern horizon, while the later Ramesside star clocks (such as those in the tombs of Ramses VI and IX) record hours by the transit of decans across a meridian system using a sighting figure. The two methods produce different decan-to-hour assignments because rising and transit move at different rates over the course of the night. Both systems coexist in the Egyptian record, and modern decanal-hour calculators can compute either depending on the source-text the user wishes to follow.

The Sothic cycle is the 1,460-year period needed for the heliacal rising of Sirius (Egyptian Sopdet, Greek Sothis) to drift back through the entire 365-day Egyptian civil year and return to the same civil date. Because the heliacal rising of Sirius marked the start of the Egyptian year and the annual Nile flood, ancient observations of "Sothis rises on civil day X" can be back-calculated astronomically to absolute dates, anchoring Egyptian chronology. Sothic calibration is therefore essential for converting decan rising tables into a modern calendar, since the decan that rises with the Sun shifts by about ten days per Egyptian century relative to the tropical year.

When Greek astronomy and astrology absorbed the Egyptian decanal system in the Ptolemaic period, the 36 decans were mapped onto the twelve zodiacal signs at three decans of ten degrees each, producing the system of decanic faces and decan rulerships found in Teucer of Babylon, Vettius Valens and the Hermetic Liber Hermetis. Later Arabic and Latin authors transmitted these decanic images and assigned them planetary lords through the Chaldean order, generating the famous "faces" used in horary and electional astrology by William Lilly and the Renaissance tradition. The decans thus survived as a triple-layer technique in which their original stellar identity (decanic asterism), zodiacal position (face) and planetary ruler all carry interpretive weight.