Sunday, April 2, 2023

Astronomy Needed to Calculate Dates of Passover & Easter

   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 Phase of Full Moon figured prominently in the annual calculation of the dates of the beginning of Passover and of Easter. (Image Sources: , By Gregory H. Revera - Own work, CC BY-SA 3.0, ) 

By Glenn A. Walsh

Reporting for SpaceWatchtower

Each year, Astronomy is used to calculate the date for the beginning of Passover in the Hebrew religion, as well as the date of Easter in the Christian religion. In both cases, the Vernal Equinox which marks the beginning of the season of Spring in Earth's Northern Hemisphere, and the Primary Lunar Phase of Full Moon, are primary parts of the calculation of the two religious festivals.

This year (2023) the Astronomical Vernal Equinox arrived on Monday, March 20 at 5:24 p.m. Eastern Daylight Saving Time (EDT) / 21:24 Coordinated Universal Time (UTC). The first Full Moon following the Vernal Equinox occurs on Thursday, April 6 at 12:34 a.m. EDT / 4:34 UTC. The second Full Moon following the Vernal Equinox will occur on Friday, May 5 at 1:34 p.m. EDT / 17:34 UTC.

This year (2023), the Hebrew festival of Passover (one of the Biblically-ordained Three Pilgrimage Festivals) runs from Wednesday, April 5, beginning at local Sunset, to Thursday, April 13, ending at local Sunset (Internet link to U.S. Naval Observatory web-site, to determine time of local Sunset, near the end of this blog-post).

This year (2023), the Christian festival of Traditional Easter runs (Traditional Easter Holy Week; last week of 40-day Traditional Lent period):

  • Palm Sunday: Sunday, April 2

  • Holy Wednesday: Wednesday, April 5

  • Maundy Thursday / Holy Thursday: Thursday, April 6

  • Good Friday: Friday, April 7

  • Holy Saturday: Saturday, April 8

  • Easter Sunday: Sunday, April 9 (Internet link to U.S. Naval Observatory web-site, to determine time of local Sunrise for Sunrise religious services, near the end of this blog-post):

  • Easter Monday (2nd day of Octave of Easter; public holiday in some nations): Monday, April 10

    This year (2023), the Christian festival of Orthodox Easter runs (Orthodox Easter Holy Week; last week of 40-day Orthodox Lent period):

  • Palm Sunday: Sunday, April 9

  • Great and Holy Wednesday: Wednesday, April 12

  • Great and Holy Thursday: Thursday, April 13

  • Good Friday: Friday, April 14

  • Holy and Great Saturday: Saturday, April 15

  • Easter Sunday: Sunday, April 16 (Internet link to U.S. Naval Observatory web-site, to determine time of local Sunrise for Sunrise religious services, near the end of this blog-post):

  • Bright Monday (2nd day of Bright Week): Monday, April 17

Calculating Date of the Beginning of Passover

Using the traditional Hebrew Calendar, which is a Lunisolar Calendar, the religious festival of Passover begins on the 15th day of the month of Nisan. In the Gregorian Calendar, this presently occurs between March 26 and April 25.

As Passover is a Spring season festival, it typically begins at local Sunset on the evening before the 15th day of the month of Nisan. Actually, in the Hebrew Calendar days officially begin at local Sunset and run until the following local Sunset. This comes from the traditional Rabbinic interpretation of the Hebrew Biblical verse, Genesis 1:5 - “There was evening and there was morning, one day.”

Usually, the 15th day of the month of Nisan begins on the evening of a Full Moon. And, this usually occurs on the first Full Moon after the Vernal Equinox, the official beginning of Spring.

In the Hebrew Calendar, the 1st day of the month of Nisan marks the beginning of the Lunar New Year. In ancient Israel, the 1st day of the month of Nisan would not commence until the Barley was ripe. This was the traditional test for the onset of Spring since, at least, the 4th century.

However, there are times when the Hebrew Calendar includes a Leap-Month. A Leap-Month is inserted into the Hebrew Calendar, to ensure the calendar follows the seasons or Moon phases.

When a Leap-Month in the Hebrew Calendar falls after the Vernal Equinox, sometimes Passover does not begin until the second Full Moon after the Vernal Equinox. The last time this occurred was in the year 2016 of the Gregorian Calendar.

Calculating Date of Easter

Currently, Easter is one of the moveable feasts, with the date determined by a Lunisolar Calendar, similar to the Hebrew Calendar. The difficulty in calculating Easter is due to the fact that our civil calendar does not match astronomical cycles.

A combination of Hebrew, Roman, and Egyptian calendars, along with local culture and customs, all contributed to the Easter calculations we have today. The Egyptians based their calendar on the cycle of the Sun, which was adopted by Roman, and later, Christian cultures. The Hebrew Calendar is based partly on the Lunar Cycle (the Islamic Calendar is also based on the Moon). The Easter calculations become complicated when both lunar and solar calendars are used, combined with the fact that different Christian sects use different mathematical formulas.

Jesus Christ's death and resurrection occurred during the Jewish holiday of Passover (which begins on the night of a Full Moon, immediately after the Vernal Equinox), according to the Christian Bible. However, this led to confusion of what date to celebrate Easter, with Christians celebrating the holiday on different dates.

There was great 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.

In the year A.D. 325, the First Council of Nicaea of the Roman Catholic Church established only two rules for the annual determination of Easter: independence from the Jewish Calendar and worldwide uniformity. The rules for actual calculation of the date of Easter took centuries to work-out.

Calculating the date of Easter caused several controversies, partly because some Christians did not want Easter to be associated with the Jewish Passover. In at least one case, violence accompanied such a controversy.

The 5th century astronomer and mathematician, Hypatia of Alexandria, Egypt, was murdered (in March of A.D. 415) by a clique of Bishop Cyril's zealots, according to the Church historian Socrates Scholasticus. According to an unconfirmed hypothesis by Canadian mathematician Ari Belenkiy, she had been attempting to calculate the date of Easter, from astronomical observations on the Vernal Equinox.

In A.D. 725, an English monk, the Venerable Bede (later known as Saint Bede), made the general rule for determining the date of Easter, by stating, “The Sunday following the full Moon which falls on or after the equinox will give the lawful Easter.” However, the Ecclesiastical rules are more specific.

Easter was determined to occur on the first Sunday, after the Ecclesiastical or Paschal Full Moon (actually determined to be the 14th day of an Ecclesiastical Lunar Month, determined by the Ecclesiastical New Moon. This date could be a couple days away from the actual, astronomical Full Moon.), which occurs on or soonest after the Vernal Equinox (which is fixed as March 21, even if this Spring Equinox occurs on March 19 or 20, which often happens).

The Ecclesiastical Lunar Month is defined as having only 29 or 30 days (relative to the Sun, the Moon orbits the Earth in about 29.53 days, known as the Synodic Month). Consequently, a Lunar Year of 12 Lunar Months has only 354 days, far shorter than the traditional Solar Year which has 365 days (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!

One of the reasons the date for Easter varies is due to the use of the Gregorian Calendar, a reform introduced by the Roman Catholic Pope Gregory XIII in 1582. 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.

Orthodox or Eastern Catholic churches often do not celebrate Easter the same week as traditional Christians; sometimes the two festivals are a few weeks apart. However, occasionally the Orthodox Holy Week does coincide with the Traditional Holy Week.

Orthodox churches continue to use the Julian Calendar to calculate Easter and other feast days including Christmas Day (January 7). By the Julian Calendar, the March 21 date of the Equinox is equated with April 3 (in our current century), when converted to the Gregorian Calendar used as the civil calendar of all nations where the Orthodox Christianity is predominant. Then, Easter always falls between April 4 and May 8 of the Gregorian Calendar. The Julian Calendar Full Moon is always several days after the actual, astronomical Full Moon, hence, the Orthodox Easter is often later, relative to the visible Moon phases, than the Western Easter.

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).

As reported in SpaceWatchtower on Sunday, 2016 March 27 (Internet link to this report near the end of this blog-post), there has been an effort for many years to fix the actual date of Easter, so Easter would no longer be a movable feast and calculating the date of Easter would be simplified. Since the 2016 report, progress has been limited in the effort to fix the date for Easter.

In November, the Catholic News Agency reported that the Roman Catholic Church and the Eastern Orthodox Church have agreed that the two denominations should celebrate Easter on a common date. Their goal is to come to a common date for Easter in 2025, which would be the 1,700th anniversary of the First Ecumenical Council of Nicea.

One possible obstacle could be tensions between different churches. After Patriarch Bartholomew confirmed in 2018 that the Ecumenical Patriarchate of Constantinople would recognize the independence of the Orthodox Church of Ukraine, the Russian Orthodox Church severed ties with the Patriarchate.

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 March or April, Astronomy will still be needed for the determination of the civil calendar for the actual date Easter falls on each year.

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 sky show explaining how Astronomy helped to calculate the date of Easter was shown to the public at several of the early planetaria, including Pittsburgh's original Buhl Planetarium and Institute of Popular Science.

Internet Links to Additional Information ---

U.S. Naval Observatory Web-Site for Computing Local Sunrise & Sunset Times [For locations outside of the United States and its territories, use geographic coordinates (Latitude & Longitude)]:

Link >>>

Passover: Link >>>

Lent Season: Link >>> 

Easter: Link >>>

Hebrew Calendar: Link >>>

Julian Calendar: Link >>>

Gregorian Calendar: Link >>>

Hypatia of Alexandria, Egypt -

Link 1 >>>

Link 2 >>>

Wimmer, A.C., Catholic News Agency. "Why Catholics and Orthodox might once again celebrate Easter on the same date." 2022 Nov. 18.

Link >>>

Related Blog-Posts ---

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

"Computus: How to Calculate the Date of Easter." Fri., 2014 April 18.

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

               Sunday, 2023 April 2.

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Glenn A. Walsh, Informal Science Educator & Communicator                                                               (For more than 50 years! - Since Monday Morning, 1972 June 12):
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Electronic Mail: < >
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SpaceWatchtower Editor / Author: Link >>>
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), America's fifth major planetarium and Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania, the fourth of only five libraries where both construction and endowment funded by famous industrialist & philanthropist Andrew Carnegie.
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 >>>

1 comment:

  1. Overall, this article sheds light on the intersection of astronomy and religion and highlights the important role that scientific calculations and observations play in determining religious observances.

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