1. What is Easter?
Easter is the principal feast of the ecclesiastical year in the Christian Church. In most parts of the world it is known as “Pascha” (or some variant of that name), which itself derives from “Pesach”, the Hebrew name for the Passover.
The Passover is a Jewish holy day to which the Christian feast is intimately linked because the trial and crucifixion of Jesus reportedly took place during preparations for celebrating the Passover. Indeed the Christian celebrations of Easter take over elements of the Jewish feast, telling of the passing of Israel through the Red Sea, and of the Paschal lamb.
2. When is Passover?
The eating of the Paschal lamb to celebrate the Passover takes place on the evening of the 14th of Nisan in the Jewish calendar, as detailed in Exodus, chapter 12:
“ And the LORD spake unto Moses and Aaron in the land of Egypt, saying, This month shall be unto you the beginning of months: it shall be the first month of the year to you. Speak ye unto all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, … And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening. … And thus shall ye eat it; with your loins girded, your shoes on your feet, and your staff in your hand; and ye shall eat it in haste: it is the LORD’s Passover. ”
The Jewish calendar is based on the cycles of the moon, with each month beginning with a new moon and lasting either twenty-nine or thirty days (a lunar month is a little over 29½ days long). Twelve lunar months come to about 354 days, so each year the Jewish calendar slips back about eleven days relative to the solar year. Approximately every fourth year, an intercalary month would be inserted to try and ensure that the new year always started at or after the vernal equinox, but this was not done according to a definite rule but at the arbitrary command of the Sanhedrin (supreme council of the Jews) based on astronomical observations. This makes it difficult to convert distant dates from the Jewish calendar into other calendar systems with certainty.
The confusion over dates is further increased because it was the practice of the Jews in Jerusalem to count their days from sundown to sundown, making some disagreement as to when the 14th of Nisan ended and the 15th began!
3. How does Easter relate to Passover?
Early Christians were divided as to the appropriate time to commemorate the death and resurrection of Jesus.
- The Eastern church based their celebrations on whichever date the Jews fixed the 14th of Nisan in each year, regardless of the day of the week it fell upon. The authority of St John and St Philip was claimed for this practice, which came to be known as quartodecimanism (on the basis that any significant schism in the church needs a word at least fifteen letters long to describe it).
- At Antioch, Easter was kept on the Sunday following the Jewish Passover (i.e. the 14th of Nisan).
- The practice at Rome and Alexandria, by contrast, was to celebrate the resurrection on the Sunday following the first full moon after the vernal equinox. This was done on the basis that the resurrection reportedly took place on a Sunday (the day after the Jewish Sabbath) following the Passover (i.e. the 14th of Nisan) which was always a full moon (because the Jewish calendar is lunar) and generally followed the equinox. The authority of St Peter and St Paul was claimed for this system.
- In parts of Gaul (modern-day France), the celebration was fixed on 27th March (in the Julian Calendar).
- The Montanist church in Asia Minor kept the feast on the Sunday following 6th April.
The First Council of Nicaea was convened in AD 325 to regularise many matters of the Christian faith and to debate and decide on the nature of the Holy Trinity. It was at this council that St Nicholas is reputed to have lost his temper with Bishop Arias of Egypt and slapped him in the face, and one lasting product of the council is the form of the Nicene Creed which is still used in Christian churches today. The council also considered the observance of the principal feasts in the ecclesiastical calendar, and decreed that the Roman practice should be observed throughout the church. This was seen as properly retaining the significance of the moon and the sun in
the timing of the commemoration of the resurrection, while also clearly establishing that Christian observances were not dependant on the Jewish calendar.
So in the modern church, Easter is celebrated on the Sunday following the first full moon after the vernal equinox, regardless of whether or not this coincides with Jewish Passover.
3. What dates can Easter fall on?
According to the rule, Easter can fall across quite a wide range of dates.
The Sunday following the first full moon after the vernal equinox can occur as early as March 22nd (but the last occasion on which it did this was in 1818, and it will not happen again until 2285).
The latest that Easter can occur is April 25th (the last occasion on which it did this was in 1943, and it will next happen in 2038). Easter was on March 23rd in 2008, and will be on April 24th in 2011.
Calculating the date on which Easter will fall in any given year is quite a complicated process. There are some interesting tables of the dates on which it can fall, and the frequency with which it does so, on this web page.
4. Why is the calculation so complicated?
Even with the basic principle for determining Easter agreed upon, there are many complications remaining.
For example, due to the distribution of leap years, the exact moment of time of the vernal equinox advances by about 6 hours each year (or jumps back about 18 hours in a leap year).
Also, the time of day at which new moons and full moons actually occur also varies, so that lunar months based on observations of the moon can vary from 28 days to 31 days in apparent extent.
Finding the date of Easter requires three different cycles to be predicted and interlocked:
- the solar cycle (motion of the earth around the sun, giving rise to the solar year)
- the lunar cycle (motion of the moon around the earth, giving rise to the lunar month)
- the diurnal cycle (rotation of the earth on its axis, giving rise to the solar day).
On top of that, to find the Sunday following the full moon needs the seven-day weekly cycle to be factored in too!
It is not surprising that the English word “computation”, along with related words such as “computer”, derive from the word Computus coined in Mediaeval times to denote the calculation of the date of Easter! Computus is derived from Latin com- together with putare and means intensive reckoning.
In fact, the establishment of reliable mechanisms to determine the date for this church feast has involved some of the finest astronomers, scientists and mathematicians over two millennia.
Further reading
The following wikipedia article is thorough, but quite hard work:
This post is adapted from an article first published in the newsletter of the Friends of St Mary's Choir, Warwick, in March 2005.
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