5 April 2015

Calculating the date of Easter

Historical note. On this day in 1815, Tambora, a 14,000-foot volcano on the Indonesian island of Sumbawa, erupted, sending a cloud of smoke and ash almost 90,000 feet into the atmosphere. The blast, report William Klingaman and Nicholas Klingaman in The Year Without Summer, was heard 800 miles away in Java. That was just the beginning. Five days later, the most violent volcanic explosion in at least 2,000 years — a blast 100 times as powerful as that of Mount St. Helens in 1980 — blew off the top 4,000 feet of the mountain, sending 100 cubic kilometers of volcanic debris into the atmosphere, where it spread around the world, blocking sunlight. The next year was the coldest since 1400.

Easter, the most important moveable feast in Christianity, the U.S. Naval Observatory, explains, “…is the first Sunday after the first ecclesiastical full moon that occurs on or after March 21.”

I recommending reading UNSO’s entire post on the date of Easter, but the following excerpt pretty well covers the heart of the matter.

The ecclesiastical rules are:

Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox;

this particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon); and

the vernal equinox is fixed as March 21.

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.

The times of the ecclesiastical full moons are not necessarily identical to the times of astronomical Full Moons. The ecclesiastical tables did not account for the full complexity of the lunar motion.

The vernal equinox has a precise astronomical definition determined by the actual apparent motion of the Sun as seen from the Earth. It is the precise time at which the apparent ecliptic longitude of the Sun is zero. (Yes, the Sun's ecliptic longitude, not its declination, is used for the astronomical definition.) This precise time shifts within the civil calendar very slightly from year to year. In the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.

The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The vernal equinox occurs at a specific date and time all over the Earth at once.

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.

The Moon was full yesterday and will be indistinguishable from full tonight. Easter falls on 27 March next year, three days after the 100th anniversary of the Easter Rebellion of 1916.