Sunday, April 10, 2022

Astronomy Used To Calculate Easter Date

   Full moon in the darkness of the night sky. It is patterned with a mix of light-tone regions and darker, irregular blotches, and scattered with varying sizes of impact craters, circles surrounded by out-thrown rays of bright ejecta.
For centuries, the Primary Moon Phase of Full Moon has figured prominently in the annual calculation of the date of Easter.
(Image Sources: , By Gregory H. Revera - Own work, CC BY-SA 3.0, )

By Glenn A. Walsh
Reporting for SpaceWatchtower 

With today (2022 April 10) being Palm Sunday, under the Traditional Christian calendar, it also represents the beginning of Traditional Easter Week, also known as Traditional Holy Week. All of the holidays related to Easter depend on the actual date of Easter each year. But, how is the date of Easter calculated?

This year, Sunday, 2022 April 17 marks the festival of Easter in the Traditional Christian calendar. However, it takes a bit of Astronomy, and knowledge of liturgical rules, to determine the Easter date each year. Once you know the date of Easter, this determines the dates of other festivals in Lent and Easter Week.

In addition to the festivals in the Traditional Lent and Traditional Easter Week, the dates for festivals in the Orthodox Lent and Orthodox Easter Week often differ.

Due to the need to use Astronomy to calculate the date of Easter and other moveable feasts, the Roman Catholic Church has supported an astronomical observatory for several centuries. The Vatican Observatory, originally established as the Observatory of the Roman College of Rome in 1774, is now located in Castel Gandolfo, Italy. The Holy See, since 1993, also operates the Vatican Advanced Technology Telescope at the Mount Graham International Observatory in southeastern Arizona.

In the 1930s and 1940s, a planetarium show explaining how Astronomy helps to calculate the date of Easter was shown to the general public at several of the early planetaria, including Pittsburgh's original Buhl Planetarium and Institute of Popular Science.

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 and the first day of the season of Lent (46 days before Easter), and during Easter Week / Holy Week: Palm Sunday (Sunday before Easter), Holy Wednesday (Wednesday before Easter), Holy Thursday (Thursday before Easter), Good Friday (Friday before Easter), and Holy Saturday (Saturday before Easter), not to mention the non-Christian Mardi Gras (Tuesday before Ash Wednesday).

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 occurs on March 20 in most years (and sometimes as early as March 19) 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 "Pascal 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 calendar!

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, sometimes drifting out of the proper season.

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 (Christian Feast of the Annunciation), while the Gregorian Calendar had moved the beginning of the year to January 1 (Christian Feast of the Circumcision of Christ).

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. An Internet link, near the end of this blog-post, provides details regarding the precise calculation of Traditional Easter, given the several liturgical rules.

By the Gregorian Calendar, Easter always falls between March 22 and April 25, within about seven days of the actual, astronomical Full Moon. The most common date for Easter, in the Gregorian Calendar, is April 19.

A large part of the eastern world, where the Orthodox Catholic Church dominates in countries such as Greece, Ukraine, and Russia, celebrate Easter and related holidays using the older Julian Calendar. In some years, the dates of Traditional Easter and Orthodox Easter coincide; the dates of related holidays also coincide in these years.

However in other years, Orthodox Easter, and related holidays, are celebrated later than Traditional Easter and related holidays. The year A.D. 2022 is one of those years. This year, Orthodox Easter will be celebrated one week later than Traditional Easter, on Sunday, 2022 April 24.

Since 1923, there have been several proposals to unify the date of Easter on one particular Sunday each year, for all Christian denominations. The most recent proposal in 2016 would set Easter as the second or third Sunday of April each year, using the Gregorian Calendar. Although no Christian denomination has approved any such change, some Church officials such as the Church of England's Archbishop of Canterbury Justin Welby had hoped a change could come within a decade.

If the date of Easter is ever fixed, Astronomy will no longer be part of this particular determination. But of course, even if Easter is designated as a certain Sunday in April, Astronomy will still be needed for the determination of the civil calendar for the actual date Easter falls on each year.

The following are the dates of the Lenten and Easter Week holidays in 2022, for both Traditional and Orthodox Christian calendars. ---

Traditional Season of Lent and Traditional Easter Week / Traditional Holy Week:

  • Tue., March 1 – Mardi Gras / Fat Tuesday / Carnival Tuesday / Shrove Tuesday / Pancake Tuesday / Pancake Day. (Day before Ash Wednesday)

  • Wed., March 2 -
    ** Ash Wednesday
    ** Beginning of Lent. (First Wednesday in Lent, 46 days before Easter Sunday)

  • April 10 to 18 – Easter Week / Holy Week. (Week of Palm Sunday through Holy Saturday; Traditionally, also including Easter Sunday, and possibly including Easter Monday)

  • Sun., April 10 – Palm Sunday. (Sunday before Easter Sunday)

  • Wed., April 13 – Holy Wednesday. (Wednesday before Easter Sunday)

  • Thur., April 14 – Maundy Thursday / Holy Thursday. (Thursday before Easter Sunday)

  • Fri., April 15 – Good Friday. (Friday before Easter Sunday)

  • Passover --- April 15, local sunset to April 23, local sunset - Jewish festival of Passover.

  • Sat., April 16 – Holy Saturday. (Saturday before Easter Sunday)

  • Actual Astronomical Full Moon --- Sat., April 16, 2:55 p.m. EDT / 18:55 UTC - Primary Moon Phase: Full Moon – Pink Moon.

  • Sun., April 17 (Sunrise in Pittsburgh: 6:39 a.m. EDT / 10:39 UTC) – Easter Sunday. [46 days after Ash Wednesday; the first Sunday after the Full Moon (the Paschal Full Moon) following the March Equinox (ecclesiastically, the Equinox is reckoned to be on March 21, even though the Equinox occurs, astronomically speaking, on March 20 in most years, and sometimes as early as March 19)]

  • Mon., April 18 – Easter Monday. (Monday after Easter Sunday)

Orthodox Season of Lent and Orthodox Easter Week / Orthodox Holy Week:

  • Mon., March 7 – Beginning of Orthodox Lent (48 days before Orthodox Easter)

  • Wed., March 9 – Orthodox Ash Wednesday (46 days before Orthodox Easter) --- SPECIAL NOTE: Generally, Eastern Orthodox churches do not observe Orthodox Ash Wednesday; however, the creation of the Antiochian Western Rite Vicariate has led to the observance of Ash Wednesday among Western Orthodox parishes.

  • Sun., April 17 - Orthodox Palm Sunday. (Sunday before Orthodox Easter Sunday)

  • Wed., April 20 - Orthodox Holy Wednesday. (Wednesday before Orthodox Easter Sunday)

  • Thur., April 21 – Orthodox Maundy Thursday / Holy Thursday. (Thursday before Orthodox Easter Sunday)

  • Fri., April 22 - Orthodox Good Friday / Holy Friday. (Friday before Orthodox Easter Sunday)

  • Sat., April 23 - Orthodox Holy Saturday. (Saturday before Orthodox Easter Sunday)

  • Sun., April 24 (Sunrise in Pittsburgh: 6:29 a.m. EDT / 10:29 UTC) - Orthodox Easter Sunday. [46 days after Orthodox Ash Wednesday; the first Sunday after the Full Moon (the Paschal Full Moon) following the March Equinox (ecclesiastically, the Equinox is reckoned to be on March 21, even though the Equinox occurs, astronomically speaking, on March 20 in most years, and sometimes as early as March 19)]

Internet Links to Additional Information ---

Details on How to Determine Date of Easter: Link >>> 

Easter: Link >>>

Lunisolar Calendar: Link >>>

Ecclesiastical Full Moon:
Link >>> 

Gregorian Calendar: Link >>> 

Julian Calendar: Link >>>

Reform Proposals for Date of Easter: Link >>> 

Vatican Observatory: Link >>>

Related Blog Post ---

"Will Christians Agree to Fix the Date of Easter?" Sun., 2016 March 27.

Link >>>

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

                 Sunday, 2022 April 10.

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Glenn A. Walsh, Informal Science Educator & Communicator:
Link >>>
Electronic Mail: < >
Project Director, Friends of the Zeiss: Link >>>
SpaceWatchtower Editor / Author: Link >>>
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh: Link >>>  Buhl Observatory: Link >>>
* Adler Planetarium, Chicago: Link >>>
* Astronomer, Educator, Optician John A. Brashear: Link >>>
* Andrew Carnegie & Carnegie Libraries: Link >>>

* Other Walsh Authored Blog & Web-Sites: Link >>>

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