How it began
Ancient Egyptians first kept a calendar based on the Moon, regulated by the seasonal appearance of the star Sirius, known as Sothis. Lunar and solar years do not fit together, and to solve that problem the Egyptians invented a schematized civil year of 365 days. Britannica describes it as established several thousand years before the common era and as the first calendar known to use a 365-day year. One reference work gives 2773 BCE as the year a standardized civic calendar was introduced, derived by counting the days between risings of Sothis.
Before it existed
The older lunar calendar followed the visible Moon and was kept in step with the seasons by the rising of Sirius. It continued to regulate religious affairs and everyday life after the civil calendar was introduced, while the civil calendar served government and administration. Egyptians later added a second, schematic lunar calendar tied to the civil year, so that three calendars ran side by side for different purposes.
How it works
The civil year had three seasons of four months each: Inundation, when the Nile overflowed the fields; Going Forth, the time of planting; and Deficiency, the time of low water and harvest. Each month had 30 days, and five extra days were added at the end of the year to make 365. Months were numbered within their season rather than named, and days were numbered within the month. A full date read as a regnal year, a numbered month of a season and a day, under a named king.
Who pushed back
The civil year was about a quarter of a day shorter than the solar year, so it fell behind by one day every four years and drifted through the seasons. Only after 1,460 years did it return to agreement with the Sirius-based calendar, a period called a Sothic cycle. The original lunar calendar was kept for agriculture because it stayed in step with the seasons.
What lasted
The civil calendar remained the Egyptian standard for dating by regnal year, and king lists recorded the total years of each reign. After Alexander's conquest the Macedonian calendar was introduced, but in Egypt it was supplanted by the Egyptian year at the end of the 3rd century BCE. A fixed year of 365 days with a simple structure, independent of the Moon, is the model that later solar calendars followed.
