Excerpts from DATASTREME DAILY SUMMARY

Friday, 21 March 1997


THE FIRST FULL MOON OF SPRING

Late Sunday also marks the occurrence of a full moon at 0445Z. The March full moon is variously called the Sap, Crow, Lenten or Pascal Moon. The first full moon following the vernal equinox is important to the timing of important religious observances in both the Jewish and Christian religions. The Pascal Moon is defined as the first moon whose 14th day comes on or after 21 March; it is used for computational purposes and the Pascal Full Moon may not always coincide with the actual full moon. Because of ecclesiastical rules associated with computation, Easter Sunday will fall on next Sunday, meaning that Christians celebrate Palm Sunday on this present Sunday The U.S. Naval Observatory has an interactive program where you can determine the date of Easter for any year since 1582, the first year following the present Gregorian calendar reform.

As the result of the lunar orbit about the earth, the moonrise for this full moon closest the spring equinox is quite different from the moonrise of its counterpart, the autumnal Harvest Moon. Unlike the slow change in the night to night time of moonrise during the week surrounding the Harvest Moon (on the order of 30 minutes), the spring full moon rises approximately one hour later each night.

Possible changes based on 1996

FULL MOON, EASTER & PASSOVER

The first full moon following the vernal equinox occurred on late on Sunday 23 March. This event is important to the timing of important religious observances in both the Jewish and Christian religions.

Passover, an important Jewish festival, begins at sundown on Tuesday 22 April 1997. Celebration of this holiday traditionally begins after sundown of the evening of the full moon (or the 14th day) of Nisan, the seventh month of the Jewish ecclesiastical year, which is based upon a lunar calendar. Consequently, Passover falls following the time of the vernal equinox.

The Christian festival of Easter will be celebrated on Sunday, 30 March 1997. Since Easter has its origin in Passover, it is celebrated on the first Sunday after the first Pascal Full Moon following the spring equinox. The Pascal Moon is defined as the first moon whose 14th day comes on or after 21 March. This Pascal Full Moon is used for computational purposes and may not always coincide with the actual full moon. Easter is a movable feast, occurring as early as 22 March and as late as 25 April. The concerns for fixing a correct date of Easter lead to astronomical studies and various calendrical reforms, including adaptation of the current Gregorian calendar.

The Eastern Orthodox Churches celebrate Easter later than the Western Christian Churches because the churches as well as many of the governments in Eastern Europe did not incorporate the Gregorian Calendar reform that Western countries had instituted in the 17th and 18th Centuries. This year the Eastern Orthodox Easter falls on Sunday 27 April 1997, following the next full moon. The difference between Easter (as well as other religious feasts, such as Christmas) in the Western and Eastern churches represents the accumulating departure between the older Julian calendar scheme, which essentially considers a year of 365.25 days, and the newer Gregorian calendar scheme that produces a year of 365.24 days.

Problems in Dating the Crucifixion

The commonly accepted reading of the Synoptic Gospels (Matthew, Mark and Luke) indicates that the Crucifixion of Jesus occurred on Nisan 15. This is based on the assumption that the Last Supper was a Passover meal on the evening that began Nisan 15. In the Gospel of John, however, the Crucifixion seems to be on Nisan 14, the Day of Preparation, when the Passover lambs were slain.

This is consistent with the Talmud, which records the Crucifixion on the day before Passover. All four Gospels agree that the event occurred on the day before the Jewish Sabbath, i.e., before nightfall on a Friday. None of the sources specifies the year, though they agree that Pontius Pilate was procurator of Judaea. This places the event in the period A.D. 26-36 (on the Julian Calendar).

From these pieces of evidence, people have speculated for centuries about the exact year of the Crucifixion.

The problem seems simple: find a Nisan 15 (or 14, if that is preferred) that ended on a Friday evening during the period A.D. 26-36. In fact, only one element of the problem is really simple. A Friday in New Testament times is just a large multiple of seven from a Friday today. Difficulties arise in determining the beginning of Nisan. Unfortunately, the Hebrew calendar of the first century A.D. is not adequately documented and must be reconstructed from fragmentary evidence. In the Hebrew calendar of that era, months began with the first sighting of the crescent Moon following astronomical New Moon, with the evening of the sighting beginning day 1 of the month. Sightings of the lunar crescent are subject to local weather conditions and the ability of the observer.

Because of these problems, a special committee of the Sanhedrin made official decisions about when to begin each month. We do not know the details of their work. However, the committee most likely judged the validity of reported sightings against predicted dates of New Moons and estimates of when the lunar crescent would become visible. If there was a stretch of bad weather, they might have ordered the month to begin 30 days following the previous beginning of the month. An occasional error of a day is quite likely. Before many months passed, however, a valid sighting would prevent accumulation of errors.

The calendar committee also had to decide when to add (intercalate) a thirteenth month into the calendar year. Since lunar months (from New Moon to New Moon) last approximately 29.5 days, a lunar year of 12 lunar months is about 354 days, which is 11 days shorter than the cycle of the seasons. To keep Nisan in the spring, a thirteenth month needs to be added about every three years. We do not know specifically how these intercalations were made. We do know that the decisions were not based exclusively on an observed or calculated date of the vernal equinox (the time at which the apparent longitude of the Sun is zero degrees). From the Bible and the Talmud we learn that the state of animal and plant life was considered, since lambs had to be mature enough for slaughter on the Day of Preparation (Nisan 14) and fruit had to be ripe enough for presentation on Nisan 16. Surviving records from the second century A.D. reveal a period when intercalations were neglected, making Nisan occur quite early. To correct this, consecutive years had thirteen months. We do not know how accurately the calendar was maintained in the first century. All this points to the fact that tables of equinoxes and Moon phases cannot alone resolve the problem. Recent studies (listed below) have tried to account for the complex problems of lunar visibility. Although both studies mention the difficulties in reconstructing the Hebrew calendar of that period, both conclude by assuming that the calendar was maintained in what we today would consider good order.

The most commonly proposed dates for the Crucifixion are April 7, A.D. 30, and April 3, A.D. 33 (on the Julian Calendar). Humphreys and Waddington conclude that the latter date is correct. Schaefer decides that their conclusion is reasonable. A definitive solution will require an independent record of the event on a fully documented calendar.

References

C. J. Humphreys and W. G. Waddington, "Dating the Crucifixion," Nature, Vol. 306, pp.743-746, 1983.

B. E. Schaefer, "Lunar visibility and the crucifixion," Quarterly Journal of the Royal Astronomical Society, Vol. 31, pp. 53-67, 1990.

The commonly stated rule, that Easter Day is the first Sunday after the Full Moon that occurs next after the vernal equinox, is somewhat misleading because it is not a precise statement of the actual ecclesiastical rules. In order that the date should be incontrovertibly fixed, and determinable indefinitely in advance, the Church constructed tables to be used permanently for calculating the age of the Moon. Easter is determined by the "ecclesiastical moon" defined by these tables, which is not strictly i dentical with the real Moon. In addition, the vernal equinox (the time at which the apparent longitude of the Sun is zero degrees) is fixed at March 21, not by the actual motion of the Sun. Moreover, the date of Easter is determined independently of any meridian of longitude, and is always the same in all time zones, unlike astronomical phenomena.

The dates of Full Moon are taken from the ecclesiastical tables, not from astronomical ephemerides. Easter Day is the Sunday that next follows the date of the ecclesiastical full moon which occurs on or next after March 21. Inevitably, the date of Easter occasionally differs from the date that would be obtained astronomically by the same rule, although when this happens it may occur only in part of the world because two dates separated by the International Date Line are always simultaneously in progress on the Earth.

In 1962, the astronomical Full Moon was on either the day after the day of the astronomical equinox, or on the same day at a later hour, according to the time zone; but ecclesiastical full moon was on March 20 throughout the world, which preceded the day of the ecclesiastical equinox. Easter was not until the Sunday that followed the next ecclesiastical full moon, which was on Wednesday, April 18.

Similarly, it may happen that the first astronomical Full Moon after the equinox either precedes or follows the ecclesiastical full moon and that the one which is first falls on Saturday. In 1954, Easter Day was April 18 throughout the world; but when astronomical Full Moon occurred, the date was already April 18 in the time zones from the International Date Line westward to the Eastern Standard Time zone, inclusive, and over this part of the world Easter was on the same Sunday as the astronomical Full Moon. If the ecclesiastical full moon falls on Sunday, but the astronomical Full Moon has already occurred, Easter is on the second Sunday after the astronomical Moon, as happened in 1876 in part of the world.

The following are dates of Easter from 1980 to 2024:

1980 April 6 1995 April 16 2010 April 4
1981 April 19 1996 April 7 2011 April 24
1982 April 11 1997 March 30 2012 April 8
1983 April 3 1998 April 12 2013 March 31
1984 April 22 1999 April 4 2014 April 20
1985 April 7 2000 April 23 2015 April 5
1986 March 30 2001 April 15 2016 March 27
1987 April 19 2002 March 31 2017 April 16
1988 April 3 2003 April 20 2018 April 1
1989 March 26 2004 April 11 2019 April 21
1990 April 15 2005 March 27 2020 April 12
1991 March 31 2006 April 16 2021 April 4
1992 April 19 2007 April 8 2022 April 17
1993 April 11 2008 March 23 2023 April 9
1994 April 3 2009 April 12 2024 March 31

Ash Wednesday is 46 days before Easter. Lent - the season that begins on Ash Wednesday and ends at Easter - is said to be 40 days long because the six Sundays that occur in this period are not considered to be part of Lent.

The date of Easter is different in the eastern (Orthodox) Christian churches. The Julian calendar, which preceded the currently-used Gregorian calendar, is the traditional basis for the ecclesiastical calendar. (The Julian calendar was still used in much of eastern Europe until the early part of the 20th century.) In a congress held in 1923, the Orthodox churches adopted a modified Gregorian calendar and decided to set the date of Easter according to the astronomical full moon for the meridian of Jerusalem. However, these changes have not been universally implemented, and a variety of practices remain among the Orthodox churches.

Computing the Date of Easter

The rule is that Easter is the first Sunday after the first ecclesiastical full moon that occurs on or after March 21.

The cycles on which the ecclesiastical moon is based can be easily programmed. The following algorithm can be used to compute the date of Easter in the Gregorian calendar system. All variables are integers and all remainders from division are dropped. The algorithm takes the year, y, and yields the month, m, and day, d, of Easter. The symbol * means multiply.

c = y / 100

n = y - 19 * ( y / 19 )

k = ( c - 17 ) / 25

i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15

i = i - 30 * ( i / 30 )

i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) )

* ( ( 21 - n ) / 11 ) )

j = y + y / 4 + i + 2 - c + c / 4

j = j - 7 * ( j / 7 )

l = i - j

m = 3 + ( l + 40 ) / 44

d = l + 28 - 31 * ( m / 4 )

The algorithm is due to J.-M. Oudin (1940) and is reprinted in the Explanatory Supplement to the Astronomical Almanac, ed. P. K. Seidelmann (1992). See Chapter 12, "Calendars", by L. E. Doggett.

Information supplied by L. E. Doggett


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Prepared by Edward J. Hopkins, Ph.D.
© Copyright, 1996, The American Meteorological Society.