The kingdom of Ancient Egypt existed for over three millennia and for much of this time was remarkable in having two different calendars in simultaneous operation. Each arose in response to different social needs and developed a distinct function. At least, this is the standard interpretation of the evidence.
The oldest Egyptian calendar was lunar. It arose in Predynastic times (prior to c. 3000 b.c.e.) from the simple need to keep agriculture in track with the seasons. Twelve lunar phase-cycle (synodic) months only amount to 354 days, so a mechanism is needed for adding an additional (intercalary) month every two to three years in order to keep the calendar in track with the seasons. It is generally supposed that in Upper Egypt (the Nile valley) the calendar was regulated by Sothis, or Sirius, whose first appearance before dawn each year (heliacal rise) coincided with the regular annual flood of the Nile, the most critical event in the agricultural year. Whenever So this was not seen until late in the twelfth month, an additional (intercalary) month was added.
Some scholars have argued that a lunar calendar had also developed independently in Lower Egypt (the Nile Delta), but that there it was regulated by observations of the sun. A myth of central importance for the Egyptians was the daily rebirth of the sun god Ra by the sky goddess Nut, who stretched from one side of the sky to the other. This legend was played out in the sky. If the Milky Way was seen as Nut herself, as has been suggested, the legend would doubtless have accounted for its shifting position with respect to the rising and setting positions of the sun, according to the time of year. This would have provided the conceptual basis for what we might too easily see as the purely pragmatic process of keeping track of the seasons by tracking the movement of the sunrise and sunset along the horizon. When the two parts of Egypt became unified into a single kingdom, these luni-stellar and luni-solar calendars would have merged.
Inscribed gray granite fragment from the “Naos of the Decades,” an Egyptian shrine dating to the fourth century B.C.E., part of a text giving astrological prognostications associated with different decades
The Early Dynastic period (c. 3000-2600 b.c.e.) was the time of the first pharaohs and the development of written records. The lunar calendar was fine for regulating agricultural activity on a local scale but became unworkable for satisfying the needs of a complex economy and state bureaucracy. Recording and regulating the movement of perishable commodities, for example, demanded absolute agreement about the date. Yet the beginning of each month, and the insertion of intercalary months, was determined by observation, and observations could differ; as a result, it was often difficult to be certain either about the month or the day in the month. The result was the development of a quite independent civil calendar used for administration purposes. This comprised twelve “months” of exactly thirty days each, divided into ten-day periods known (rather confusingly, given that the word is commonly taken to mean “ten years”) as decades, followed by a five-day period known as the additional or epagomenal days, making a total of 365. Once the calendar was set up, it was defined indefinitely; there was no need to regulate it by observations of nature. It was so fit for its purpose that it survived well beyond the end of ancient Egypt itself and was still being used by astronomers in medieval times.
It may seem surprising to suggest that the lunar calendar did not die out when the civil calendar was introduced. Yet this is precisely what has generally been believed for many years, following the work of the great Egyptian scholar R. A. Parker in the 1940s. According to this view, the purpose of the lunar calendar became the regulation of religious observances. Responsibility for determining the correct month and day within this calendar, and with it the timing of religious festivals, became the responsibility of certain priests. At some stage, and possibly as early as Old Kingdom times (c. 2600-2100 b.c.e.), Egyptians started to use the heliacal rise of other stars to help regulate the calendar and to develop the system of decans. These were thirty-six stars or star groups whose heliacal risings occurred in succession, roughly ten days apart—in other words, marking successive decades (ten-day periods) in the civil calendar. This meant that the two calendars could easily be synchronized.
Only recently has a different view emerged, proposed by the Spanish archaeoastronomer Juan Antonio Belmonte. This is that different Egyptian calendars did not in fact coexist. In Belmonte’s view, sun observations were used initially to establish the 365-day calendar, but thereafter the dates of lunar-related religious festivals were simply established within this calendar, rather as Christians determine the annual date of Easter to this day.
Whatever the outcome of this debate, it is clear that the decans also heralded a hugely important innovation: they could be used as “clocks” to mark the passing of time during the night. Instead of focusing just on the heliacal rise, that is, on the decan rising immediately before dawn, one simply had to view the risings of successive decans through the night. On this basis, the Egyptians were the first to divide the night into approximately equal time intervals. Theoretically, at the equinoxes, when sunrise and sunset are exactly twelve hours apart, eighteen ideally placed decans should rise between sunset and sunrise at intervals of 40 minutes. In practice, however, the length of the night varies through the year, and twilight renders the first and last risings invisible. Furthermore, it could not have been possible to find stars to use as decans that were precisely evenly spaced across the sky. Finally, the Egyptian kingdom stretched sufficiently far from north to south for latitude to make a difference. The most decans that could actually be seen in one night, during the longest nights of the year around the winter solstice, was twelve. This fact, it is widely believed, led to the concept of the night being divided into twelve “hours,” whose length varied through the year, but which were nonetheless a precursor to the modern concept of dividing the day-night cycle into twenty-four equal hours. There is a scholarly consensus on the identification of several of the decans, but the identification of others remains extremely speculative.
The Egyptian civil calendar was simple, useful, and all-pervasive. It worked unfailingly for almost three millennia, and although it gradually slipped with respect to the seasons (historical sources confirm that the civil new year once more coincided with the heliacal rising of Sirius in c.e. 139), this never seems to have created a problem during any particular epoch. Not until the third century b.c.e. was any attempt made to add leap years.