Easter and the Calendar

The modern-day scientist is motivated by any number of human emotions. Chief among those is curiosity, an unquenchable desire to know how the world works. In the words of Einstein, “I want to know God’s thoughts.” These words from one whose God was nature itself, the God of Spinoza, nonetheless point to the reasons that figures of the more distant past wanted to figure things out.

Our Gregorian calendar is named after Pope Gregory XIII, and his motivation was that the date of Easter had drifted from the time of year in which it was celebrated in the early church.

The Julian calendar which this new calendar replaced had a leap year of 366 days every four years, thereby assuming that a solar year was exactly 365.25 days long. The actual length is 365.2422 days. This inaccuracy had led to a drift for the date of the equinoxes and solstices of ten days by the 16^th century.

The Gregorian calendar retains the leap years except for century years such as 1800 or 1900. The one other refinement it adds is to retain the leap day of February 29^th in century years that are divisible by 400. 2000 was therefore a leap year while 2100 will not be. This results in a year that is 365. 2425 days long, very close to the actual figure.

The small remaining inaccuracy causes a drift in the dates of the equinoxes and solstices over a century that is “adjusted” by the non-leap century years. The image below shows the date of the June solstice (summer in the northern hemisphere, winter in the southern hemisphere) on the y-axis and the Gregorian year on the x-axis.

In 1903 the June solstice occurred at 10:04 am EST on June 22^nd. In 2096 it will occur at 1:31 am EST on June 20^th. (I am assuming a more enlightened future will have eliminated daylight saving time by then.)

Picture By BasZoetekouw; spelling corrections and revision of subtitle by User:Gerry Ashton on 14 September 2008. – Own work; The data was generated by Astrolabe (https://sourceforge.net/projects/astrolabegenera), which uses the algorithms described in Jean Meeus’s “Astronomical Algorithms” (ISBN 978-0943396613). The data have an error of less than 2.6 minutes., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4730687

So what IS the date of Easter? It is, as Ernest Hemingway could have told us, a moveable feast. For western Christianity it is the first Sunday after the first full moon after the ecclesiastical March equinox, which is fixed at March 21^st regardless of the actual date. The earliest possible date is therefore March 22^nd and the latest is April 25^th.

Happy Easter!