The Coptic calendar follows the sun and calculates the days, seasons, and solar years according to the earth's orbit around the sun.
As science advanced, the Egyptian Christians began to record the difference between one moon's course and another, the average time of a moon's course, and the time between the appearance of a new crescent and the following crescent, which is known as the lunar month.
The Coptic lunar calendar was based on rules set by the Greek astronomer Meton in the fifth century BC, which state that every 19 solar years is equivalent to 235 whole lunar months without any fractions.
The Copts have used this rule since the third century AD to determine Easter as the Sunday following the first full moon after the spring equinox.
Westerners applied these calculations to the Roman calendar. As a result, all Christian festivals were held at the same time, until 1582 when the Westerners adjusted their calendar to that of Pope Gregory XIII.
The Coptic calendar follows the same system as that of the ancient Egyptian calendar and remains an important reference. Because it is accurate in its forecasts of weather conditions, the seasons, agriculture, and the Nile flooding, farmers still use the Coptic calendar .