The Egyptian calendar

The story

The river decides the year

For the Egyptians the year did not begin with winter, but with the Nile. Every summer, because of the rains that fell on the mountains of Ethiopia, the river swelled and left its bed. The water covered the fields for weeks and, when it withdrew, it left a layer of black, fertile silt. The Egyptians called this moment akhet, “the flood”, and considered it the start of the year. Then, when the land re-emerged, came the season of sowing and growth, peret; finally the last, shemu, was the season of harvest, when the river was at its lowest. Three seasons of four months each, then, and not four like ours.

There were twelve months, of thirty days, and each month was divided into three weeks of ten days. That made 360 days. The Egyptians knew perfectly well that the year is a little longer, so they added five days at the end, outside the months. These were the days “upon the year”, considered the birthday feast of some gods, including Osiris, Isis and Seth. The total came to 365 days. This is the Egyptian civil calendar, and everything suggests it was already in use in the Old Kingdom, but the date of its invention is uncertain: some place it shortly after 3000 BC, others around 2800 or 2700 BC.

The star that announces the flood

The Egyptians watched the sky carefully. They noticed that there was a very bright star, our Sirius, which disappeared for about seventy days and then came back into view, for a brief moment, at dawn, just before the Sun. This event is called the heliacal rising of Sirius. Since it fell, more or less, in the days when the Nile began to rise, the star, which the Egyptians called Sopdet and identified with the goddess Isis, was seen as the announcement of the flood. For this reason a year based on Sirius could serve as a guide for farmers.

A calendar that slips

There was, however, a problem. The true year lasts 365 days and about six hours (a quarter of a day more). The Egyptian calendar had no day added every four years, like our leap years. Each year it fell six hours behind the sun and the seasons. After four years a day was missing, after one hundred and twenty years a whole month. After 1,461 years the calendar had gone all the way round and again matched the seasons. Scholars call this period the Sothic cycle, from Sothis, the Greek name for Sirius.

Since the difference built up slowly, it did not change much for a farmer. For scribes and priests, however, the New Year festival could fall in the middle of winter. To know when the flood was really about to arrive, they went on watching the sky and measuring the level of the river with nilometers, stepped wells with a graduated scale dug along the bank.

In 238 BC a decree of Canopus, issued by the priests under King Ptolemy III, proposed adding one day every four years. The reform was not applied. It was only in 26–23 BC, under the emperor Augustus, that a year with the extra day was adopted at Alexandria.

The priests' clock

The day, too, was divided by an original system. At night the Egyptians watched the passage of thirty-six groups of stars, called decans, each of which rose at regular intervals. Twelve of them were used for the night, and from this came the idea of dividing the night into twelve hours. The day was also divided into twelve hours, making twenty-four in all. These were unequal hours, because they changed in length with the seasons. To measure them people needed sundials and water clocks, the oldest of which to have reached us comes from the temple of Karnak, from about 1400 BC.

From Egypt to Rome and to us

The other peoples of the time, such as the Mesopotamians and later the Greeks, used calendars based on the Moon, with months of 29 or 30 days that did not match the year and had to be corrected often. The Egyptian calendar, with its simple rule, was much more convenient, and the Greek astronomers of Alexandria used it for their calculations. When Julius Caesar, in 46 BC, wanted to reform the muddled Roman calendar, he had the help of an astronomer from Alexandria, Sosigenes, and took from Egypt the solar year of 365 days with one extra day every four years. This is the Julian calendar (see the card on Caesar). In 1582 Pope Gregory XIII corrected it slightly, and it is the calendar we use today.

The Egyptian calendar still lives, in a modern form, in the Coptic Church of Egypt and in Ethiopia, where the year still has twelve months of thirty days and a thirteenth month of five or six days.