Egyptian calendar

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 establishment of the Alexandrian or Coptic calendar by Augustus.

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.

[4] 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.

[15] 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.

[19] 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.

[21] 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.

[29] 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.

[54] It was probably based upon astronomical observations of Sirius[15] whose reappearance in the sky closely corresponded to the average onset of the Nile flood through the 5th and 4th millennium BC.

[14][p] 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),[56] 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.

[60] The regular months were grouped into Egypt's three seasons,[59] which gave them their original names,[61] and divided into three 10-day periods known as decans or decades.

The importance of the calendar to Egyptian religion is reflected in the use of the title "Lord of Years" (Nb Rnpt)[65] for its various creator gods.

Sirius itself, about 40° below the ecliptic, follows a Sothic year almost exactly matching that of the Sun, with its reappearance now occurring at the latitude of Cairo (ancient Heliopolis and Memphis) on 19 July (Julian), only two or three days later than its occurrence in early antiquity.

[72] Although it is certain the Egyptian day began in the morning, another four years are shifted depending on whether the precise start occurred at the first light of dawn or at sunrise.

[87] The record and celebration of Sirius's rising would also vary by several days (equating to decades of the cycle) in eras when the official site of observation was moved from near Cairo.

Following Alexander the Great's conquest of the Persian Empire, the Macedonian Ptolemaic Dynasty came to power in Egypt, continuing to use its native calendars with Hellenized names.

In 238 BC, Ptolemy III's Canopus Decree ordered that every 4th year should incorporate a sixth day in its intercalary month,[90] honoring him and his wife as gods equivalent to the children of Nut.

As the personal ruler of Egypt, he also imposed a reform of its calendar in 26 or 25 BC, possibly to correspond with the beginning of a new Callipic cycle, with the first leap day occurring on 6 Epag.

Alan Gardiner proposed that an original calendar governed by the priests of Ra was supplanted by an improvement developed by the partisans of Thoth.

Discovered in modern-day Thebes, it dates from the Ramesside Period and acts as a guide to which days were considered lucky or unlucky.

Nevertheless, the different copies of the calendars are remarkably consistent with each other, with only 9.2% of the determinations of adversity or fortuitousness being due to a defined textual reason.