The Egyptians appear to have used a purely lunar calendar prior to the establishment of the solar civil calendar[1][2] in which each month began on the morning when the waning crescent moon could no longer be seen.[3] 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.[2] The lunar calendar divided the month into four weeks, reflecting each quarter of the lunar phases.[4] Because the exact time of morning considered to begin the Egyptian day remains uncertain[5] 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,[6] there is no sure way to reconstruct exact dates in the lunar calendar from its known dates.[5] 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.[7] 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.[5]
Parker and others have argued for its development into an observational and then calculated lunisolar calendar[8] 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.[1] No evidence for such a month, however, exists in the present historical record.[9]
A second lunar calendar is attested by a demotic astronomical papyrus[11] 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.[12] 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,[13] using it to show the cycle was developed to correspond with the new moon around 357BC.[14] 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.[15] 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.[16]
The days of the lunar month — known to the Egyptians as a "temple month"[10] — 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".[17]
Grafton, Anthony Thomas; et al. (1985), "Technical Chronology and Astrological History in Varro, Censorinus, and Others", The Classical Quarterly, Vol. XXXV, No. 2, pp. 454–465.
Krauss, Rolf; et al., eds. (2006), Ancient Egyptian Chronology, Handbook of Oriental Studies, Sect. 1, Vol. 83, Leiden: Brill.
Luft, Ulrich (2006), "Absolute Chronology in Egypt in the First Quarter of the Second Millennium BC", Egypt and the Levant, Vol. XVI, Austrian Academy of Sciences Press, pp. 309–316.
Neugebauer, Otto Eduard (1939), "Die Bedeutungslosigkeit der 'Sothisperiode' für die Älteste Ägyptische Chronologie", Acta Orientalia, No. 16, pp. 169 ff. (in German)
O'Mara, Patrick F. (January 2003), "Censorinus, the Sothic Cycle, and Calendar Year One in Ancient Egypt: The Epistemological Problem", Journal of Near Eastern Studies, Vol. LXII, No. 1, Chicago: University of Chicago Press, pp. 17–26.
Calendrica Includes the Egyptian civil calendar with years in Ptolemy's Nabonassar Era (year 1 = 747 BC) as well as the Coptic, Ethiopic, and French calendars.
Civil, ver. 4.0, is a 25kB DOS program to convert dates in the Egyptian civil calendar to the Julian or Gregorian ones
