by Tim Tompson*

Easter Sunday will soon be upon us.

In the year 325 A.D. the First Council of Nicaea defined the date for Easter as the first Sunday following the first Full moon after the Vernal equinox. But astronomically selected dates can move around; e.g., the vernal equinox can happen on 20 March as well as 21 March, and the phases of the moon are not tied either to the civil calendar nor to the equinoxes. So, for the purposes of calculating the date for Easter, the Roman Catholic church defines its own equinox as always happening on 21 March, and they use their own “ecclesiastical full moon”, which by definition always occurs on the 14th day of the ecclesiastical lunar month on the ecclesiastical lunar calendar (which assumes by definition that 19 tropical years equals 235 synodic months exactly [the correct number is 234.997]). In this way, Roman Catholic Easter always falls in the window of 22 March to 25 April.

Roman Catholics use the Gregorian calendar, which was finalized in 1582 for the explicit purpose of returning the date of Easter to the same date it had when the First Council of Nicaea met. In the time between 325 and 1582, the vernal equinox had slipped backwards through the civil calendar to 11 March instead of 21 March, which it was in 325. So when the Gregorian calendar replaced the Julian calendar in 1582, ten days were skipped over, which moved Easter back to 21 March. And by the trick of skipping leap days in years ending in 00, unless they are evenly divisible by 400, the length of the average civil calendar year is shortened from 365.25 days (that is one “tropical year”) to 365.2425 days, the end result of which is that the civil calendar will fall behind the seasons by about one day come the year 3200 (a problem easily solved by skipping the leap day in 3200).

The actual time it takes to go from one vernal equinox to the next is 365.24219878125 days, which should result in an accumulated difference between the seasons and the civil calendar of 3 days, 17 minutes, 33 seconds over 10,000 years. But the mean tropical year is decreasing by 0.53 seconds per 100 years (a slow tidal transfer of energy from sun to Earth), and the mean length of day is decreasing by 0.0015 seconds per 100 years (a slow tidal transfer of energy from Earth to moon). That’s why the calendar can lose a whole day in only about 1200 years. One could cleanup the next few thousand years by skipping the leap day in the year 3200, keep the leap day in 3600 and 4000, and skipping the leap day in 4500 & 5000.

Eastern Christians (mostly the Eastern and Greek Orthodox churches, Eastern Catholic and Coptics) use the old Julian calendar, so they celebrate Easter basically a month later than do the Romans, in the window between 4 April to 8 May.

There are various good reasons for having a civil calendar that is locked to the seasons. But the one that has been most important has proven to be the need to have Easter fall on a fixed time of the civil calendar year.

* The writer is a physicist retired from NASA’s Jet Propulsion Laboratory. Among his main personal interests are astronomy, chess, languages and linguistics, and military history.


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