Friday, April 18, 2014

Computus: How to Calculate the Date of Easter

This month's Full Moon, which came at a time of a Total Eclipse
of the Moon, is also considered, ecclesiastically, the Paschal
Full Moon. (Image Source: NASA)

By Glenn A. Walsh
Reporting for SpaceWatchtower

This Sunday marks the festival of Easter in the Christian calendar. However, it takes a bit of Astronomy, and knowledge of liturgical rules, to determine the Easter date each year.

In general, each year Easter falls on the Sunday following the Full Moon (called the "Paschal Full Moon") which follows Spring's Vernal Equinox in the Northern Hemisphere. From this date are set several other days in the Christian calendar: Ash Wednesday (46 days before Easter), and during Holy Week: Palm Sunday (Sunday before Easter), Holy Thursday (Thursday before Easter), Good Friday (Friday before Easter), and Holy Saturday (Saturday before Easter).

So, the determination of Easter sets a considerable part of the Spring Season for the Christian calendar. However, the determination of Easter is not that simple.

First, ecclesiastical definitions are not always the same as astronomical definitions. The ecclesiastical Spring Equinox is defined as always occurring on March 21 (used under the Gregorian calendar reform by the Roman Catholic Church since 1583). The astronomical Vernal Equinox often occurs on March 20, except in the far Eastern Hemisphere when it does occur the next calendar day.

The "Pascal Full Moon" is not always the same as the regular Full Moon of the month. The Roman Catholic Church defined the ecclesiastical Full Moon as occurring 14 days after the beginning of the ecclesiastical lunar month (determined by the ecclesiastical New Moon).

To further complicate matters, the ecclesiastical lunar month was defined as having 29 or 30 days. A lunar year of 12 lunar months, by the ecclesiastical definition, has a total of 354 days, far shorter than the traditional solar year which is defined as having 365 days (and 366 days during a Leap Year). When the difference in the ecclesiastical lunar year and the solar year reaches or exceeds 30 days, then an additional lunar month is added to the ecclesiastical lunar year!

The reason for all of these special rules came from dissatisfaction expressed by many Christians, in the 3rd and 4th centuries, regarding previous methods of establishing the date of Easter. Originally, they simply used the Jewish festival of Passover and set Sunday of the Passover week as Easter. Some did not like, what they perceived as, the general disorderly state of the Jewish calendar. Others were upset that, by using the Jewish calendar, Easter was sometimes celebrated before the Vernal Equinox, the beginning of Spring.

The First Council of Nicaea in A.D 325 was the first time the Roman Catholic Church officially addressed this issue. It was agreed that Christians should use a calendar to determine the Easter date separate from the Jewish calendar. However, little else was agreed-to at that time. It took several centuries before a method to compute the Easter date was common throughout the world's Catholics. For a while a computation method developed in Alexandria, Egypt was the most accepted computation.

With the Gregorian calendar reform of 1582, one computation method of determining Easter was established throughout the Roman Catholic Church. The Gregorian Calendar was established as a refinement of the Julian Calendar, which had incorrectly calculated the length of the year by 0.002 per cent. Although this seems a minor problem, it became a major problem as the actual dates of church holidays, particularly Easter, had been drifting.

With the English Reformation between 1532 and 1537, England and English colonies did not comply with the Gregorian calendar reform of 1582, thus remaining with the Julian Calendar. The British Empire did not accept the Gregorian Calendar until 1752. Hence, George Washington was born on February 11 in 1731 by the "Old System" (Julian Calendar), but his birthday is now celebrated on February 22 by the Gregorian Calendar (also, George Washington is now considered to have been born in the year 1732; in the "Old System" calendar, the year 1732 did not begin until March 25).

So now with the Gregorian Calendar in general agreement world-wide, again in general, Easter falls on the Sunday following the first Full Moon after the Vernal Equinox in the Northern Hemisphere. The following link provides details regarding the precise calculation of this holiday, given the several liturgical rules.

More Details - Link >>>

Related Blog Post ---

Total Lunar Eclipse Early Tue. Morning w/ Web-Cast  (2014 April 14):

 Link >>>

Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friend of the Zeiss.

2014: 75th Year of Pittsburgh's Buhl Planetarium Historic Zeiss II Planetarium Projector at Pittsburgh's original Buhl Planetarium and Institute of Popular Science.

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