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: Wikipedia.org , By Gregory H. Revera - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11901243 )
By Glenn A. Walsh
Reporting for SpaceWatchtower
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.
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).
year (2023), the Christian festival of Traditional Easter runs
(Traditional Easter Holy Week; last week of 40-day Traditional Lent period):
Sunday: Sunday, April 2
Wednesday: Wednesday, April 5
Thursday / Holy Thursday: Thursday, April 6
Friday: Friday, April 7
Saturday: Saturday, April 8
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):
day of Octave of Easter; public holiday in some nations): Monday,
year (2023), the Christian festival of Orthodox Easter runs (Orthodox Easter Holy Week; last week of 40-day Orthodox Lent period):
Sunday: Sunday, April 9
and Holy Wednesday: Wednesday, April 12
and Holy Thursday: Thursday, April 13
Friday: Friday, April 14
and Great Saturday: Saturday, April 15
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):
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
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.
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.
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
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.
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
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.
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.
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).
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!
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
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.
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.
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).
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.
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.
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
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 >>> https://aa.usno.navy.mil/data/RS_OneDay
Passover: Link >>> https://en.wikipedia.org/wiki/Passover
Lent Season: Link >>> https://en.wikipedia.org/wiki/Lent
Easter: Link >>> https://en.wikipedia.org/wiki/Easter
Hebrew Calendar: Link >>> https://en.wikipedia.org/wiki/Hebrew_calendar
Julian Calendar: Link >>> https://en.wikipedia.org/wiki/Julian_calendar
Gregorian Calendar: Link >>> https://en.wikipedia.org/wiki/Gregorian_calendar
Hypatia of Alexandria, Egypt -
Link 1 >>> https://www.change.org/p/canada-s-parliament-commemorating-the-first-female-astronomer-hypatia-of-alexandria
Link 2 >>> https://en.wikipedia.org/wiki/Hypatia
Wimmer, A.C., Catholic News Agency. "Why Catholics and Orthodox might once again celebrate Easter on the same date." 2022 Nov. 18.
Link >>> https://www.catholicnewsagency.com/news/252851/why-catholics-and-orthodox-might-once-again-celebrate-easter-on-the-same-date
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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Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of
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Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh: Link >>> http://www.planetarium.cc Buhl Observatory: Link >>> http://spacewatchtower.blogspot.com/2016/11/75th-anniversary-americas-5th-public.html
* Adler Planetarium, Chicago: Link >>> http://adlerplanetarium.tripod.com
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