Triangle and Ordinary Decans

Definition

Inside an Egyptian diagonal star table — the night-hour chart painted in Middle Kingdom coffins — the 36 decans split into two working groups. The ordinary decans are the main run of stars that mark the hours across the year's 36 ten-day "weeks." The triangle decans are a smaller extra set, named for the triangular block they form in one corner of the table, added to cover the five leftover days at the year's end.

In Tradition

The split exists because the Egyptian civil year is 365 days, not a tidy 360. Thirty-six decades of ten days each cover only 360 days; the remaining five "days upon the year" (the epagomenal days) fall outside that scheme. Egyptologists read the triangle decans as the clock-makers' fix — extra hour-stars stitched in so the night could still be timed during those five odd days.

In Practice

In the table, the ordinary decans are numbered 1 to 36 and fill the main body of the grid, one set of twelve hour-stars per ten-day decade. To keep the clock working through the five epagomenal days, the Egyptians added a column of triangle decans — labelled A to L plus an extra one, M, in Neugebauer and Parker's scheme — so that twelve triangle decans could mark those final hours. The triangular shape comes from how new triangle decans had to be slotted in, each rising about halfway between the last and first ordinary decans, as the year ran out. Symons notes a practical wrinkle: on several of the best-preserved coffin lids only about 34 distinct ordinary decans actually appear, with the small gap filled by repeating the opening pair (tmat hrt). The diagonal tables themselves do not visually flag which decans are "triangle" and which are "ordinary" — that distinction is recovered from other Egyptian star lists where the two classes appear apart.

Historical Origin

The triangle/ordinary distinction is reconstructed from the Middle Kingdom diagonal star tables (c. 2055-1650 BCE) on coffin lids. The standard analysis is Neugebauer & Parker, *Egyptian Astronomical Texts* Vol I (1960); the two decan classes are detailed in Symons, *Ancient Egyptian Astronomy* (1999), Section A, and in Belmonte & Lull, *The Astronomy of Ancient Egypt* (2018), §3.1.1, who list the twelve triangle decans in the 40th column of the ideal diagonal clock.

Further Reading

  • Sarah Symons, Ancient Egyptian Astronomy
  • Otto Neugebauer & Richard A. Parker, Egyptian Astronomical Texts, Volume I: The Early Decans
  • Juan Antonio Belmonte & José Lull, The Astronomy of Ancient Egypt