ASTRONOMY ( HERCHEL)

THE HISTORY OF EASTER:

A SONG TO THE MOON

Brian Robertson, William Herschel Society, on 10 March 2001

A performance of Dvorak's A Song to the Moon by Lucia Popp from the opera Resalka preceded the talk, which was sub-titled The Computus Paschalis, the Jewish procedure for calculating the date of the Passover.

The Spring (vernal) equinox, always the most important of the four, was named Eaostre after the Norse goddess and this was adopted as Easter by the Anglo-Saxons, under the guidance of Augustine, using the Roman method of calculating the date of it. But the Celtic-based churches in the north of England continued to use the older method of calculation until the Synod of Whitby in 664.

The date of the vernal equinox wanders because of the variations in the orbits of the earth and the moon, their inter-action (they together revolve round their common centre of gravity) over the years, and the precession of the equinoxes.

The date of Easter Day is calculated because it was originally based on the Jewish lunar calendar, (the Jews say the moon was created to count the days) which had to be integrated with the solar-based secular calendar of the Romans, instituted by Julius Caesar in 44 BC (the Julian calendar). The lunar year has 12 or 13 months in a solar year, and there are at least five measures of the number of days in a lunar month:

Synodical (the time between successive new moons) is the visible and common period of 29.53 days

Anomalistic (the time between successive perigees of the moon) is 27.55 days

Sidereal (the time between successive passages over a meridian) is 27.32 days

Nodical is 27.26 days

Tropical is 27.32 days

In 19 solar years there are 7 lunar years of 13 months and 12 of 12 months (235 total) the Metonic cycle, which was used by the Jews.

Three pieces of evidence from the New Testament, in various Gospels, have to be reconciled to determine the date of Easter Sunday (Christ's Resurrection):

• it occurred in the (lunar) month of NISAN

• the eve of the Passover is the 14th of that month

• Christ's resurrection took place on a Sunday (the first day of the week)

However, inaccuracies in the Jewish calendar and lack of records confused the true date within a few centuries.

In the fourth century, Constantine, one of two joint Roman Emperors, whose mother Helena was a Christian, used Christianity to consolidate his power. He won a battle over the other joint Emperor, Maxentius, became sole Emperor, and in 325 called a conference at Nicea to settle disputes between various bishoprics, and to integrate Easter into the Roman calendar. He decreed that Easter Day is the first Sunday after the first full moon after the vernal equinox (March 21st).

From that time, monks calculated the date by the elaborate formula shown below. Easter Day is always on one of the 35 days from 22 March to 25 April. The monks kept Greek learning alive during the `Dark Ages' but the church also persecuted and murdered scientists who, after Copernicus showed that the earth was not the stationary centre of the
universe (which the Catholic Church only accepted in 1992), disagreed with its opinion.

Calculating the Date of Easter Day

Let a = year divided by 19, keep only the remainder

Let b = year divided by 100, neglect remainder

Let c = year divided by 100, keep only remainder

Let d = b divided by 4, neglect remainder

Let e = b divided by 4, keep only remainder

Let f = (b+8) divided by 25, neglect remainder

Let g = (b-f+1) divided by 3, neglect remainder

Let h = 19(a+b-d-g+15) divided by 30, keep only remainder

Let i = c divided by 4, neglect remainder

Let k = c divided by 4, keep only remainder

Let L = 32(2e+2)(i-h-k) divided by 7, keep only remainder

Let m = (a+11)(h+22)L, divided by 451, neglect remainder

Then Easter month =

(h+1-7)(m+114) divided by 31, neglect remainder

[3 = March , 4 = April]

Then p = (h+1-7)(m+114), divided by 31, keep only remainder

EASTER DAY = p+1 in Easter month

Discussion

Should the date of Easter now be fixed?

A Bill was introduced to do so in 1928 but has never been passed. It would have placed Easter between 8 and 15 April, which is not `fixed'. Many people considered it time to fix a Spring holiday unconnected with the Christian church's celebration of Christ's Resurrection, particularly in view of the multi-faith nation we have become. The speaker was reluctant to lose the historic and astronomical connection. It was pointed out the Scottish and some northern English holiday periods were disconnected from church holy days, especially Christmas. Also, the Orthodox Church has still not adopted the Gregorian calendar, and celebrates Christmas on our 6 January.

Who decided when the first Sunday should be?

This led to a confused and lengthy discussion involving the Zoroastrians, who decided, about 3000 BC, that there were seven days in a week because there were seven days in each phase of the moon and that the first should be used for prayer; the astronomers who calculate time from 6 am on 1 January 27?? BC for an astronomical calendar and Universal Time; and the proposal of Arthur C. Clark that time should be the same everywhere on earth, which led to some confusion about the International Date Line and round-the world travel.

(An article in The William Herschel Society's magazine `Speculum' will provide a more detailed account of the history of the calculation of Easter Day.)