Ancient Egyptian Civil Calendar

The civil calendar the pharaonic bureaucracy kept was exactly three hundred sixty five days, no more and no less, with no leap day to correct it: three seasons of four months each, Akhet the inundation, Peret the emergence, and Shemu the harvest, every month a fixed thirty days, giving three hundred sixty days closed by five epagomenal days appended at the year's end. Egyptian mythology held these five extra days to be the birthdays of five siblings born to the sky goddess Nut, Osiris, Horus the Elder, Set, Isis and Nephthys in order, a genealogy recorded by Plutarch centuries after the calendar itself was already ancient and reflected in earlier Egyptian sources as well.

Because the true solar year runs about a quarter day longer than three hundred sixty five, the civil calendar drifted against the seasons by a full day every four years and completed an entire cycle back to its starting alignment only once every roughly fourteen hundred sixty years, the interval astronomers call the Sothic cycle after Sopdet, the star Sirius, whose heliacal rising the calendar had originally been set to track before the drift began. A parallel lunar calendar, kept for temple ritual rather than civil administration, ran alongside this fixed count for the whole of its history.

The consequence of having no leap day was not a minor inconvenience but a slow unmooring of the calendar from the reality it was built to track. Over the centuries, the month the calendar called the start of the inundation season could fall in any actual season at all, since the error compounds relentlessly and the names Akhet, Peret and Shemu described increasingly little about the weather or the Nile on the ground.

Ptolemy III attempted a fix in the Decree of Canopus of 238 BCE, proposing an extra day every fourth year on the later Julian pattern, but Egyptian priestly and administrative resistance meant the reform was never generally adopted in his lifetime. It took Augustus's annexation of Egypt and his calendar reform of 26 BCE, imposing the Julian leap year rule onto the old Egyptian month structure, to finally fix the drift, and that fixed version is the direct ancestor of the Coptic calendar Egyptian Christians still keep. The unreformed civil calendar itself is no longer kept by anyone: it belongs to history rather than to any living community, though its month names, its thirty day structure and its five epagomenal days survive almost unchanged in the Coptic and Ethiopian calendars that inherited its shape while finally correcting the flaw that had made it drift.