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Choosing the date of Easter and how Western, Eastern celebrations are on the same day this year
Determining the proper date for Easter has been a more complex issue, and sometimes a more contentious one, than many Christians realize. The complexities of astronomy have played a major role in this. - photo by Deseret Connect
For most premodern peoples, the entire cosmos was sacred, because it was both created by God and filled with divine power and presence. God (or the gods) had organized the heavens as markers for measuring time and as signs of divine will. This idea is reflected in the Hebrew creation story: And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years (Genesis 1:14). The lights of the firmament had thus been created in part so mankind could measure time.

Most ancient peoples shared broadly similar views and, hence, became dedicated sky-watchers. Through close observation, ancient priests recognized predictable repetitive patterns in the heavens. The most obvious is day and night, but many other patterns were noticed by the ancients and catalogued into calendars.

The problem for these ancient sky-watchers was that heavenly movements did not conform to regular patterns. The relative length of night and day not only changed throughout the year but also regionally from north to south. The year consists of 365.2422 days. The sidereal lunar month is 27.321 days, but this can likewise vary because of movement of the Earth.

These irregularities caused endless consternation among ancient astronomers. Had I been present at the Creation, the 13th-century King Alfonso X of Castile remarked ironically upon studying the complexities of Ptolemaic astronomy, I would have given some useful hints to God for the better ordering of the universe.

Many different ancient calendars were developed, attempting to reconcile the irregularities of the heavens.

Ancient Jews faced similar problems in creating their sacred calendar. They developed a complex lunisolar calendar that tried to reconcile the differences between lunar months and the solar year by adding a 13th month to the lunar calendar every three or four years, so as to keep their seasonal festivals in the proper season (e.g., Passover in the spring). Different Jewish sects, however, offered conflicting interpretations of the calendar, which yielded different dates for Passover. The current Jewish calendar dates Passover on the 15th day of the month of Nissan, meaning the first full moon after the vernal equinox.

Thus, by the time of Jesus Christ, the dating of Passover, and hence Easter, was already complicated. Todays Jewish rabbinic calendar is likewise somewhat different from that of the first century A.D. Hence the uncertainty among modern scholars regarding the precise year and day of Christs crucifixion.

The earliest Christians, mainly converts from Judaism, commemorated the crucifixion on the 14th of Nissan, according to the Jewish calendar of that period (known as Quartodecimanism fourteenism). A wide variety of customs on the dating of Easter developed thereafter in early Christendom. With the rise of the Imperial Church under Constantine, the Council of Nicaea in 325 determined that Easter must always fall on a Sunday, which permanently severed it from the Jewish calendar. Other Christian groups, such as the Celtic Christians of Ireland and early Syriac Christians in the Middle East, continued with Easter more closely tied to the Jewish calendar.

Based on the interpretations of the Council of Nicaea, medieval Catholics established the date of Easter as the first Sunday after the full moon following the vernal equinox. Easter thus became a moveable feast, since it does not fall on the same day each year. This dating was likewise later adopted by Protestants after the Reformation, and it remains the dominant way of dating Easter.

To complicate matters further, however, the medieval dating of Easter used the old Roman civic calendar created by Julius Caesar. An improvement on the earlier Roman calendar, this Julian calendar set the solar year as 365.25 days hence our leap year every four years. Unfortunately, the actual solar year is slightly less, at 365.2425 days.

Thus, over the course of 1,500 years, the Julian calendar used to establish Easter fell out of synchronization with the seasons by 10 days. To resynchronize the calendar with the seasons, Pope Gregory XII created the Gregorian calendar in 1582, which was eventually accepted throughout the West.

The Greek Orthodox church, however, retained the older Julian calendar, and thus Orthodox and Western Easter are now generally celebrated on separate days. This year, however, presents one of the rare occasions when Western and Eastern Easter fall on the same Sunday. All the worlds Christians will unite in commemorating the risen Christ on the same day.
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