Sirius and the Kemetic calendar

A year of 365 days

Kemet's civil calendar had twelve months of thirty days each, grouped into three seasons of four months: Akhet, the flood; Peret, the growing season; and Shemu, the harvest. Five extra days, the "days upon the year", were added at the end, giving 365. The Egyptologist Richard Parker showed in 1950 that Kemet actually kept three calendars: this civil one for government and business, and two lunar calendars for religious festivals.

The civil year was a quarter of a day shorter than the true solar year. With no leap day, it slipped one day against the seasons every four years. Over centuries, New Year's Day wandered through every season, which is why it is often called the "wandering year". A decree issued at Canopus under Ptolemy III tried to add a sixth extra day every fourth year, but the change took hold only under Roman rule.

Sopdet, the star of the new year

Sirius is the brightest star in the night sky. For part of each year it is hidden in the sun's glare. Its first reappearance in the east just before sunrise, its heliacal rising, came in early summer, about when the Nile began to rise. Kemetic texts call the star Sopdet (Greek Sothis), a goddess later joined with Auset (Isis). Her return was celebrated as a new year and a sign that the life-giving flood was coming. Parker read an early dynastic label as naming Sopdet as "opener of the year" and the inundation.

Using Sirius to date the pharaohs

Because the civil year drifted, a date written in that calendar for the rising of Sirius pins down a span of years. In 1904 the historian Eduard Meyer built a system of "Sothic dating" on this idea, and it still underlies the standard dates for the Middle and New Kingdoms. Two records matter most:

• A papyrus from the Middle Kingdom town of Illahun (el-Lahun) reports a coming rising of Sirius in the reign of Senusret III.
• A note on the back of the medical Ebers papyrus (shown above) records a rising of Sirius in year 9 of Amenhotep I, often placed in the 1500s BCE.

What is debated

That Kemet kept a 365-day year and honoured the rising of Sirius is Documented. How precisely those records can date the kings is Debated. The result depends on where the star was observed, since each degree of latitude shifts the date of first sighting by about a day, which moves the answer by about four years. Scholars also disagree about which king's reign the Ebers note belongs to, whether it records an observation or a calculation, and whether it uses the civil or a lunar calendar. The researcher David Lappin (2020) reviews these problems and notes that if the records were tied to the star's disappearance in May instead of its rising, the Sothic dates would move more than two centuries later. Many Egyptologists still use the conventional Sothic dates, cross-checked against recorded dates of the moon.