The mathematical reckoning of time is fundamentally a cultural and civilizational matter related to astronomy, in which the ancient Egyptians excelled. This extended to their daily lives and the festival seasons. Calendars have varied across different civilizations geographically, historically, and socially, to the extent that each region, society, or group of countries had its calendar—one that did not necessarily have the same starting point, name, or division. It is entirely separate from faith and inherited doctrine in every civilization. It is purely an astronomical and mathematical matter, no more and no less, but over time, standards, values, and traditions have been added to it in different places, making it well-known and well-established.

The Greek historian Herodotus (who lived about three centuries before Christ) wrote about the Coptic (Egyptian) calendar, stating:

“The ancient Egyptians were the first to devise the calculation of the year. They divided it into twelve parts based on their knowledge of the stars, and it appears to me that they are more skillful than the Greeks (Hellenes), for the Egyptians calculated the month as thirty days and added five days to the year so that the season would revolve and return to its starting point.”

The Egyptians, 4,200 years before Christ, divided the year into 12 constellations across three seasons—the inundation, the cultivation, and the harvest—with each season lasting four months. They also divided the year into weeks and days, the day into 24 hours, the hour into 60 minutes, the minute into 60 seconds, and the second itself into 60 divisions.

The Coptic calendar is stellar-sidereal, meaning it is linked to the cycle of the star Sirius, the brightest star in the Canis Major (Greater Dog) constellation. The Egyptians observed the heliacal rising of this star—when it first became visible at dawn before sunrise—aligned with the nose of the Great Sphinx. This event marked the feast of their great deity, coinciding with the arrival of the Nile floodwaters at Memphis, near Giza.

They calculated the length of the year based on the cycle of this star as 365 days. However, they noticed that their fixed festivals only returned to their proper astronomical positions once every 1,460 years. They then divided 365 by 1,460 and found that the result was 1/4 of a day. Thus, they added 1/4 of a day to the length of the year, making it 365 days and a quarter—adding a full extra day every fourth year (leap year). This correction ensured that their festivals remained in their proper astronomical positions, aligning with the length of the day and night. This adjustment was made when Egyptian priest-astronomers convened (around three centuries before Christ) at Canopus (modern-day Abu Qir near Alexandria). They discovered this discrepancy and decided to correct it in the famous decree issued by Ptolemy III, known as the Decree of Canopus.

The months of the Coptic year are, in order: Tout, Baba, Hatoor, Kiahk, Toba, Amshir, Baramhat, Baramouda, Bashans, Baona, Apip, and Mesra, in addition to the Little Month called “Nasie” that consists of five days or six days in a leap year. These months are still used in Egypt today, not only in the liturgical calendar but also popularly, especially in agriculture.

The Copts adopted the year in which Emperor Diocletian took the throne (284 AD) as the starting point of their Church calendar because he persecuted and killed hundreds of thousands of Copts (around 800,000 martyrs). This calendar was therefore named the Era of the Martyrs, Anno Martyrum.

The Julian Calendar

The Julian calendar was named after Julius Caesar, the Roman ruler who instructed an Egyptian astronomer from the famous Alexandrian School, named Sosigenes, to establish March 25 as the vernal equinox. Sosigenes modeled the Roman year after the Egyptian year, making it exactly 365 days and 6 hours (a quarter-day). He then adjusted the months by adding extra days so that the ordinary year would be 365 days, and the leap year would be 366 days. This calendar remained in use both in the East and the West until Pope Gregory XIII of Rome, based on the advice of astronomers, modified it in 1582, skipping 10 days by making October 4 immediately followed by October 15. This change became known as the Gregorian Reform.

The Revised (Gregorian) Calendar

Pope Gregory XIII noticed a discrepancy in the date of the vernal equinox compared to its position at the time of the Council of Nicaea (325 AD)—amounting to a shift of ten days. The equinox, which had been on March 21 (corresponding to 25 Baramhat) in 325 AD, had drifted forward to March 11 by 1582 AD. Since this was an astronomical issue, not a theological one, Pope Gregory XIII consulted astronomers, who explained that the problem was due to the method of calculating the length of the year. The actual time taken for the Earth to orbit the Sun is 365 days, 5 hours, 48 minutes, and 46 seconds. However, the Julian calendar had assumed a year length of 365 days and 6 hours, causing a discrepancy of 11 minutes and 14 seconds per year. This accumulated to 1 day every 128 years. From the Council of Nicaea (325 AD) to the time of Pope Gregory XIII (1582 AD), this error had accumulated to 10 days, shifting the date of all fixed feasts. This difference has now increased to 13 days in our present time.

The Gregorian calendar is the most widely used in the modern world. It was first adopted by Catholic countries, including Rome, France, Spain, and Portugal. It was later introduced in England in 1752, Japan in 1872, and Egypt in 1875 during the reign of Khedive Ismail. As for Eastern Churches, they continued to rely on the Julian system, which was based on the ancient Egyptian calendar. The Julian leap-year rule simply added one extra day every four years without exception. This resulted in the current 13-day difference from the time of the Council of Nicaea until now, which is why the Coptic Nativity (Christmas) on 29 Kiahk now falls on January 7 in the Gregorian calendar.

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