Coptic calendar

The Coptic calendar, also called the Alexandrian Calendar, is used by the Coptic Orthodox Church. It is based on the Ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the Ancient Egyptian calendar was introduced at the time of Ptolemy III (Decree of Canopus, in 238 BC) which consisted in the intercalation of a 6th epagomenal day every fourth year. However, this reform was opposed by the Egyptian priests, and the idea was not adopted until 25 BC, when the Roman Emperor Augustus formally reformed the calendar of Egypt, keeping it forever synchronized with the newly introduced Julian calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.

The date of Easter

According to Christian tradition, Jesus died at the ninth hour (that is, the canonical hour of nona or 'noon' in Middle English - 3:00 pm) of the first full day of Pesach, when that day fell on a Friday; and arose from the dead at or by the first (canonical) hour of that Sunday. The day of Pesach (Pascha or Passover, Nisan 15), is always at the first or second full moon following the vernal equinox. At the First Ecumenical Council, held in 325 at Nicaea, it was decided to celebrate Easter on the Sunday following the so-called Paschal full moon. The Paschal full moon is an arithmetical approximation to the first full moon after the vernal equinox. It may be expressed as follows in terms of the so-called Golden number (G) and Century term (C):

Related Topics:
Jesus - Pesach - Nisan 15 - Ecumenical Council - 325 - Nicaea

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• Paschal full moon (PFM) = (19 April, or 50 March) - (C+11G) mod 30

Except in two cases where the PFM is one day earlier than this, namely:

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• When (C+11G) is 0 modulo 30, PFM = 18 April (not 19 April).
• When (C+11G) is 1 modulo 30, and G=12, PFM = 17 April (not 18).

The Golden number (G) is the same for both Julian and Gregorian computations, but the Century term is constant (C = +3) in Julian computations:

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:G ,=, 1 + (Y mod 19) in year Y (Julian or Gregorian).

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:C ,=, leftlfloor H / 4 ight floor - H + leftlfloor (H+11) / 25 ight floor with H = leftlfloor Y / 100 ight floor (Gregorian year Y)

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C is -4 from 1583 to 1699, -5 from 1700 to 1899, -6 from 1900 to 2199, -7 from 2200 to 2299 etc... As the Sunday following the PFM, Easter is one week after the PFM when the PFM happens to fall on a Sunday. One must work with the Julian calendar (C = +3) to find when Easter is celebrated by Orthodox churches.

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At the Council of Nicaea, it became one duty of the bishop of Alexandria to determine the exact dates of Easter and to announce it to the rest of the Christian churches. This duty fell on this officate because of the erudition at Alexandria he could draw on. The precise rules to determine this are very involved, but Easter is usually the first Sunday after a full moon occurring no sooner than March 21, which was the actual date of the vernal equinox at the time of the First Council of Nicaea. Shortly before Julius Caesar reformed the calendar, the vernal equinox was occurring on the "nominal" date of March 25. This was abandoned at Nicaea, but the reason for the observed discrepancy was all but ignored (the actual tropical year is not quite equal to the Julian year of 365¼ days, so the date of the equinox keeps creeping back in the Julian calendar).

Related Topics:
Council of Nicaea - Alexandria - March 21 - Vernal equinox - Julius Caesar - March 25 - Tropical year

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See also: Computus

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