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Easter is an annual festival observed throughout the Christian world. The date for Easter shifts every year within the Gregorian Calendar. The Gregorian Calendar is the standard international calendar for civil use. In addition, it regulates the ceremonial cycle of the Roman Catholic and Protestant churches. The current Gregorian ecclesiastical rules that determine the date of Easter trace back to 325 CE at the First Council of Nicaea convened by the Roman Emperor Constantine. At that time the Roman world used the Julian Calendar (put in place by Julius Caesar).
The Council decided to keep Easter on a Sunday, the same Sunday throughout the world. To fix incontrovertibly the date for Easter, and to make it determinable indefinitely in advance, the Council constructed special tables to compute the date. These tables were revised in the following few centuries resulting eventually in the tables constructed by the 6th century Abbot of Scythia, Dionysis Exiguus. Nonetheless, different means of calculations continued in use throughout the Christian world.
In 1582 Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian calendar and produced new Easter tables. One major difference between the Julian and Gregorian Calendar is the "leap year rule". See below for a description of the difference. Universal adoption of this Gregorian calendar occurred slowly. By the 1700's, though, most of western Europe had adopted the Gregorian Calendar. The Eastern Christian churches still determine the Easter dates using the older Julian Calendar method.
The usual statement, that "Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox", is not a precise statement of the actual ecclesiastical rules. The full moon involved is not the astronomical Full Moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical Moon.
The ecclesiastical rules are:
resulting in that Easter can never occur before March 22 or later than April 25. The Gregorian dates for the ecclesiastical full moon come from the Gregorian tables. Therefore, the civil date of Easter depends upon which tables - Gregorian or pre-Gregorian - are used. The western (Roman Catholic and Protestant) Christian churches use the Gregorian tables; many eastern (Orthodox) Christian churches use the older tables based on the Julian Calendar.
In a congress held in 1923, the eastern 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, a variety of practices remain among the eastern churches.
There are three major differences between the ecclesiastical system and the astronomical system.
Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.
For example, take the year 1962. In 1962, the astronomical Full Moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox. The ecclesiastical full moon (taken from the tables), however, occurred on March 20, before the fixed ecclesiastical equinox at March 21. In the astronomical case, the Full Moon followed its equinox; in the ecclesiastical case, it preceded its equinox. Following the rules, Easter, therefore, was not until the Sunday that followed the next ecclesiastical full moon (Wednesday, April 18) making Easter Sunday, April 22.
Similarly, in 1954 the first ecclesiastical full moon after March 21 fell on Saturday, April 17. Thus, Easter was Sunday, April 18. The astronomical equinox also occurred on March 21. The next astronomical Full Moon occurred on April 18 at UT=5h. So in some places in the world Easter was on the same Sunday as the astronomical Full Moon.
Principal Solar Calendars:
The Julian Calendar was introduced by Julius Caesar in 45 BCE and introduced a simple leap year rule: insert an extra day every four years. The Julian Calendar eventually standardized on 21 March as the date of the vernal equinox. Although this leap year rule is a simple one, it is does not produce a precise match to the solar year. Over the centuries the date of the astronomical vernal equinox slowly drifted away from the date of 21 March. The ecclesiastical rules to compute the date of Easter defined 21 March as the date of the vernal equinox. The Gregorian Calendar resulted from a perceived need to reform the calculation method for the dates of Easter. Nonetheless, the Julian Calendar and variations of it are still in use by some groups to set the dates for liturgical events.
The Gregorian Calendar has become the internationally accepted civil calendar. The leap year rule for the Gregorian Calendar differs slightly from one for the Julian Calendar. The Gregorian leap year rule is: Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year. The Gregorian dates for Easter are computed from a set of ecclesiastical rules and tables.
Principal Lunar Calendars:
The Islamic Calendar is a purely lunar calendar in which months correspond to the lunar phase cycle. Thus the twelve months of the Islamic Calendar systematically shift with respect to the months of the international civil calendar. The cycle of twelve months regresses through the seasons over a period of about 33 years. For religious purposes, Muslims begin each month with the first visibility of the lunar crescent after conjunction. For civil purposes a tabulated calendar that approximates the lunar phase cycle is often used.
The astronomical date and time of each New Moon can be computed exactly; however, the time an observer first sees that young Moon cannot be computed exactly. The time the Moon first becomes visible after the New Moon depends on many factors. The various effects are the geometry of the Sun, Moon, and natural horizon; the width and surface brightness of the crescent; the absorption of the Moon's light and the scattering of the Sun's light in the Earth's atmosphere; and the physiology of human vision. These things all change very rapidly. Our information page on Crescent Moon Visibility and the Islamic Calendar includes information on the difficulties associated with visual sightings of the crescent Moon. Her Majesty's Nautical Almanac Office computes the time of New Moon and provides information sheets that give the date of earliest visibility of the new crescent Moon for each lunar month for a selection of cities in the UK and around the world.
The Hebrew Calendar is a lunisolar calendar based on calculation rather than observation. Its current form dates from about 359 CE. This calendar is the official calendar for the State of Israel, although variations on this calendar exist. The dates for Passover for this calendar are computed from a set of defined rules.
The National Calendar of India is a formalized lunisolar calendar in which leap years coincide with those of the Gregorian calendar. The Gregorian Calendar is used for administrative purposes. The Indian religious calendars require calculations of the motions of the Sun and Moon. Tabulations of the religious holidays are prepared by the India Meteorological Department and published annually in The Indian Astronomical Ephemeris. Many local variations exist.
The Chinese Calendar is a lunisolar calendar based on calculations of the positions of the Sun and Moon. Since this calendar uses the true positions of the Sun and Moon, its accuracy depends on the accuracy of the astronomical theories and calculations.
Special purpose calendars:
There are also many special purpose calendars. Some are based on abstract, perpetually repeating cycles of no astronomical significance. Some calendars are regulated by observations (not computed or tabulated times) of celestial events. Some calendars are codified in written laws; others are transmitted by oral tradition. Many of these calendars provide dates for religious events and depend upon, for example, the occurrence of a specific religious, cultural, or agricultural event. They may or may not tie this to a date on the international civil calendar. They may or may not tie this to other astronomical events such as the astronomical vernal equinox. Those calendars are outside the scope of this web site.