Egyptian calendar

The ancient Egyptian calendar – a civil calendar – was a solar calendar with a 365-day year. The year consisted of three seasons of 120 days each, plus an intercalary month of five epagomenal days treated as outside of the year proper. Each season was divided into four months of 30 days. These twelve months were initially numbered within each season but came to also be known by the names of their principal festivals. Each month was divided into three 10-day periods known as decans or decades. It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work.

Because this calendrical year was nearly a quarter of a day shorter than the solar year, the Egyptian calendar lost about one day every four years relative to the Julian calendar. It is therefore sometimes referred to as the wandering year (Latin: annus vagus), as its months rotated about one day through the solar year every four years. Ptolemy III's Canopus Decree attempted to correct this through the introduction of a sixth epagomenal day every four years but the proposal was resisted by the Egyptian priests and people and abandoned until the decree by Augustus in 25 BC that established the Alexandrian or Coptic calendar. The introduction of a leap day to the Egyptian calendar made it equivalent to the Julian calendar, although (like the latter) it continues to diverge from the Gregorian calendar at the turn of most centuries.

This civil calendar ran concurrently with an Egyptian lunar calendar which was used for some religious rituals and festivals. Some Egyptologists have described it as lunisolar, with an intercalary month supposedly added every two or three years to maintain its consistency with the solar year, but no evidence of such intercalation before the 4th century BC has yet been discovered.

History

Prehistory

Current understanding of the earliest development of the Egyptian calendar remains speculative. A tablet from the reign of the First Dynasty pharaoh Djer (c. 3000 BC) was once thought to indicate that the Egyptians had already established a link between the heliacal rising of Sirius (Ancient Egyptian: Spdt or Sopdet, "Triangle"; Ancient Greek: Σῶθις, Sôthis) and the beginning of their year, but more recent analysis has questioned whether the tablet's picture refers to Sirius at all. Similarly, based on the Palermo Stone, Alexander Scharff proposed that the Old Kingdom observed a 320-day year, but his theory has not been widely accepted. Some evidence suggests the early civil calendar had 360 days, although it might merely reflect the unusual status of the five epagomenal days as days "added on" to the proper year.

With its interior effectively rainless for thousands of years, ancient Egypt was "a gift of the river" Nile, whose annual flooding organized the natural year into three broad natural seasons known to the Egyptians as:

1. Inundation or Flood (Ancient Egyptian: Ꜣḫt, sometimes anglicized as Akhet): roughly from September to January.
2. Emergence or Winter (Prt, sometimes anglicized as Peret): roughly from January to May.
3. Low Water or Harvest or Summer (Šmw, sometimes anglicized as Shemu): roughly from May to September.

As early as the reign of Djer (c. 3000 BC, Dynasty I), yearly records were being kept of the flood's high-water mark. Otto E. Neugebauer noted that a 365-day year can be established by averaging a few decades of accurate observations of the Nile flood without any need for astronomical observations, although the great irregularity of the flood from year to year and the difficulty of maintaining a sufficiently accurate Nilometer and record in prehistoric Egypt has caused other scholars to doubt that it formed the basis for the Egyptian calendar.
Note that the names of the three natural seasons were incorporated into the Civil calendar year (see below), but as this calendar year is a wandering year, the seasons of this calendar slowly rotate through the natural solar year, meaning that Civil season Akhet/Inundation only occasionally coincided with the Nile inundation.

Lunar calendar

The Egyptians appear to have used a purely lunar calendar prior to the establishment of the solar civil calendar in which each month began on the morning when the waning crescent moon could no longer be seen. Until the closing of Egypt's polytheist temples under the Byzantines, the lunar calendar continued to be used as the liturgical year of various cults. The lunar calendar divided the month into four weeks, reflecting each quarter of the lunar phases. Because the exact time of morning considered to begin the Egyptian day remains uncertain and there is no evidence that any method other than observation was used to determine the beginnings of the lunar months prior to the 4th century BC, there is no sure way to reconstruct exact dates in the lunar calendar from its known dates. The difference between beginning the day at the first light of dawn or at sunrise accounts for an 11–14 year shift in dated observations of the lunar cycle. It remains unknown how the Egyptians dealt with obscurement by clouds when they occurred and the best current algorithms have been shown to differ from actual observation of the waning crescent moon in about one-in-five cases.

Parker and others have argued for its development into an observational and then calculated lunisolar calendar which used a 30 day intercalary month every two to three years to accommodate the lunar year's loss of about 11 days a year relative to the solar year and to maintain the placement of the heliacal rising of Sirius within its twelfth month. No evidence for such a month, however, exists in the present historical record.

A second lunar calendar is attested by a demotic astronomical papyrus dating to sometime after 144 AD which outlines a lunisolar calendar operating in accordance with the Egyptian civil calendar according to a 25 year cycle. The calendar seems to show its month beginning with the first visibility of the waxing crescent moon, but Parker displayed an error in the cycle of about a day in 500 years, using it to show the cycle was developed to correspond with the new moon around 357 BC. This date places it prior to the Ptolemaic period and within the native Egyptian Dynasty XXX. Egypt's 1st Persian occupation, however, seems likely to have been its inspiration. This lunisolar calendar's calculations apparently continued to be used without correction into the Roman period, even when they no longer precisely matched the observable lunar phases.

The days of the lunar month — known to the Egyptians as a "temple month" — were individually named and celebrated as stages in the life of the moon god, variously Thoth in the Middle Kingdom or Khonsu in the Ptolemaic era: "He ... is conceived ... on Psḏntyw; he is born on Ꜣbd; he grows old after Smdt".

Civil calendar

The civil calendar was established at some early date in or before the Old Kingdom, with probable evidence of its use early in the reign of Shepseskaf (c. 2510 BC, Dynasty IV) and certain attestation during the reign of Neferirkare (mid-25th century BC, Dynasty V). It was probably based upon astronomical observations of Sirius whose reappearance in the sky closely corresponded to the average onset of the Nile flood through the 5th and 4th millennium BC. A recent development is the discovery that the 30-day month of the Mesopotamian calendar dates as late as the Jemdet Nasr Period (late 4th-millennium BC), a time Egyptian culture was borrowing various objects and cultural features from the Fertile Crescent, leaving open the possibility that the main features of the calendar were borrowed in one direction or the other as well.

The civil year comprised exactly 365 days, divided into 12 months of 30 days each and an intercalary month of five days, which were celebrated as the birthdays of the gods Osiris, Horus, Set, Isis, and Nephthys. The regular months were grouped into Egypt's three seasons, which gave them their original names, and divided into three 10-day periods known as decans or decades. In later sources, these were distinguished as "first", "middle", and "last". It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work. Dates were typically expressed in a YMD format, with a pharaoh's regnal year followed by the month followed by the day of the month. For example, the New Year occurred on I Akhet 1.

The importance of the calendar to Egyptian religion is reflected in the use of the title "Lord of Years" (Nb Rnpt) for its various creator gods. Time was also considered an integral aspect of Maat, the cosmic order which opposed chaos, lies, and violence.