Kemetic decans and star clocks

Telling time by the stars

The Kemetic year had 36 ten-day "weeks" plus five extra days. For each week, astronomers chose a star or small star group that rose on the eastern horizon just before dawn. These are known today by the Greek name decans, from the word for ten. Because the sky turns through the night, a series of decans rises one after another after dark. Watching which one had just come up told an observer the hour.

Historians of astronomy, following Neugebauer and Parker, think that the number of decans seen to rise on a short summer night, about twelve, is how the night came to be divided into twelve hours. Joined with twelve hours of daylight, this became the 24-hour day used worldwide today.

Star clocks on coffin lids

The oldest surviving decan lists are painted on the inside of wooden coffin lids from the First Intermediate Period and the Middle Kingdom, from about 2100 BCE, many of them from the town of Asyut. They are set out as a grid now called a diagonal star table: columns for the weeks of the year, rows for the hours of the night. Because each decan moves up one row every ten days, its name runs diagonally across the grid.

Otto Neugebauer and Richard Parker described the tables on twelve coffin lids and one monument in 1960. The astronomer Sarah Symons of McMaster University later found that the number of published tables has more than doubled. Writing with Elizabeth Tasker in Scientific American (2015), she explains that the names of most decans are known, but which stars they were is still uncertain, because we do not fully know how the tables were observed and compiled. Symons and Tasker suggest that inside a coffin the table served the dead person as a map of the sky, where they hoped to live on among the stars.

Senenmut's ceiling

The most famous Kemetic sky map is on the ceiling of the tomb of Senenmut (TT 353) at Deir el-Bahri, an official of the female king Hatshepsut, about 1479–1458 BCE. The Metropolitan Museum's excavators found it in the 1920s. The ceiling divides the sky into a northern half and a southern half. The figures painted there are constellations or protective deities, and below them are twelve circles, one for each month (Metropolitan Museum). The original tomb is closed to the public; the Met displays a painted copy made by Charles K. Wilkinson (shown above).

Later history

Decans kept appearing in tombs and temples for some two thousand years. In the Greco-Roman period they were shown together with the twelve zodiac signs that had come from Babylon, as on the Dendera zodiac, whose outer ring carries all 36. This history is Documented; only the identification of individual decan stars remains open.