Easter and astronomy

The date for Easter can be confusing and is historically controversial in that it pits Western and Eastern Christian theology. Easter is what is known as a moveable feast calculated to be the first Sunday after the first full moon [Paschal] after the spring equinox [that would be the Northern spring equinox or for the Southern hemisphere, the autumn equinox]. Also consider that the phrase "full moon" is vague and is relative to your position on earth. Confused yet?...well, factor in those leap years and the computation mathematics required to evaluate the point where the sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect.

The fulcrum for all of this is Jesus and calendars.

The events surrounding the death, burial, and resurrection of Jesus occurred during the Jewish Passover celebrations. Obviously, this association is of great theological import. So the early church was keen to keep a close association between Easter and Passover. But the Jewish calendar is a lunar calendar, fixing the celebration on the 15th day of the month of Nisan. This places Passover in the spring in the Northern Hemisphere. Thus, Easter is always in the spring.

However, after Constantine, as Christianity came to be centered on Rome, the Roman solar calendar came to dominate the celebrations of the church. It was difficult to manage a lunar calendar for religious feasts [Passover and Easter] and a solar calendar for civil activities. And, the 15th of Nisan doesn't always fall on the same day of the week. This complicated things for the church who wanted to associate the first day of the week [Sunday] with Easter. So the early church made a compromise. It switched to a solar calendar but kept Easter in the spring, keeping it seasonally associated with Passover [and still, vestigially, following the phases of the moon], as well as locking it onto a Sunday.

This did not solve anything. This early solution was based on the Julian calendar. Julius Caesar introduced this calendar to Rome after seeing the advantages of the Egyptian solar year. The Julian calendar gives us a year of 365 1/4 days. Which is remarkably close to the actual solar year. But the match is not exact. A solar year is closer to 365 and 1/4th days long. So with the Julian calendar you are drifting against the solar year by 1/4 day each year. This drift is not much to notice on a year to year basis but over a century your calendar is drifting about 25 days, almost a full month. Eventually, if you date Easter by the Julian calendar Easter ends up being in the dead of winter. Now this Julian drift can and was dealt with by adding in days here and there to catch the calendar up with the sun. Only precise astronomy [establishing the exact length of the solar year] could help. Eventually, all the frustrations and science fell into the lap of Pope Gregory XIII who, in 1582, created the Gregorian calendar. It is the Gregorian calendar that gives us the leap years which correct every four years for the 1/4 day drift [but again, it is much more complicated than this as a solar year isn't exactly 365 1/4th days]. The Gregorian calendar now governs most of the world. And this solved nothing either. The Eastern Orthodox church didn't go along with the Gregorian calendar reforms. They stayed with the Julian calendar. Consequently, to this day, there are two Easters in Christendom, each celebrated on different days.

In case you want to know...Easter this year will be on April 4th.