Calculating Easter and Passover

Calculating Easter is not an easy task. It used to be the Sunday of the Jewish Passover week. In 325 A.D. it was decided that Easter would be after the first full moon that came after the Spring Equinox.

The Week of Unleavened Bread is symbolic of the Jewish slaves being liberated from Egypt and there was not enough time to allow the bread to rise. Passover or Pesakh, is a reminder of the Jewish liberation in 1,300 B.C., which starts on the fifteenth day of the Hebrew month of Nisan. This is when the barley is ripe and it is a fertility benchmark.

Early Christian churches had their Easter celebration about this time. Christians used to ask their Jewish neighbors when the Week of Unleavened Bread was as the date changes annually. Christians would celebrate Easter the Sunday of the same week.

Easter became a celebration that came with spring’s rebirth of agricultural life, in the Northern hemisphere. At the Council of Nicaea in 325 A.D. it was determined that Easter would be the first Sunday after the first full moon, after the Spring Equinox. Then count backwards from that date 40 days and that is the start of Lent.

If the first full moon is on a Sunday, Easter will be a week later. The celebration will always be between March 22 and April 25 according to the Gregorian calendar. However, there are the Orthodox Christian churches that use the Julian calendar. There are 19 possible dates for the Easter Full Moon to fall on between March 21 and April 18. The Julian calendar and the Gregorian calendar are not in sync with each other by 13 days, the calculation of the Paschal full moon is different as well.

Lunar months are 29 or 30 days long, making the lunar year shorter than the solar year by 11 days. The synodic month is the average length of the lunar month and is exactly 29.530588853 days long. There are 12 synodic months in a lunar year. The extra days in the solar year, are called epacts. To find the corresponding day to a solar year day, add the epact to the date in the solar year. When the epact gets up to 30 and higher, an intercalary month has to be added to the lunar year, then 30 is subtracted from the epact and the cycle resets to zero.

This is the Metonic cycle that repeats every 19 years which explains why there are only 19 possible days for the Paschal full moon, March 21 to April 18. However, the calendar cycle is not exact, so every 19 years there is a lunar leap day to make up the difference. The Eastern Orthodox churches do not have a lunar leap day so that makes the difference between the ecclesiastical full moon and the astronomical full moon. Orthodox Easter is not always 13 days after the non-orthodox Christian churches because the start date depends on whether the ecclesiastical moon is synchronized with the astronomical full moon. Two religions whose celebrations are determined by two different moons.

By Jeanette Smith

Sources: