Easter is a movable feast, meaning it doesn’t occur on a fixed day of the year. It can occur anywhere from March 22 to April 25. Roughly speaking, Easter is the first Sunday following the full moon that occurs after the Spring equinox. However, the ecclesiastical definitions of equinox and full moon are not the astronomical ones, and different sects of Christianity have used different definitions over the years, resulting in a squabble or two. Perhaps a bit more than a squabble. Factions invented names for rival factions; and, as Socrates Scholasticus tells it, Bishop John Chrysostom booted some of his Easter-calculation opponents out of the early Christian church. By the 6th century, the papal authorities had legislated the calculations for Easter, By the twelfth century, they had to face the fact that Easter had drifted badly.
Fortunately, around that time, the ancient Greek mathematicians had just been translated to Latin (Ptolemy’s Almagest in particular), thanks to the learned and ever helpful Arabs. By the time of the Renaissance, travel arrangements and event catering meant that the popes needed to plan for Easter celebrations many years in advance. The popes understood that since the moon and sun revolved around the earth (God’s plan for astronomy) solar events would take place at different times around the world. So Easter could fall on a different date in London than in Ethiopia.
This was a mess. Science, something previously rather unwelcome in the church, was the only way to solve the messy problem of predicting Easter. And the popes happened to have money to throw at the problem. They suddenly became the world’s largest backer of scientific research – well, targeted research, one might say. As John Heilbron, author of “The Sun in the Church” puts it:
The Roman Catholic Church gave more financial support to the study of astronomy for over six centuries, from the recovery of ancient learning during the late Middle Ages into the Enlightenment, than any other, and, probably, all other, institutions. Those who infer the Church’s attitude from its persecution of Galileo may be reassured to know that the basis of its generosity to astronomy was not a love of science but a problem of administration. The problem was establishing and promulgating the date of Easter.
The nasty part of the calculation was determining the exact time at which the sun returned to the same equinox. This measurement would require a large observatory with a small hole in the roof and a flat floor where one could draw a long north-south line. The scientists would mark a spot on the floor where the sun shone through ever day at noon. When the spot returned to the same point on the north-south line, you had the crux of the Easter calculation.
Coincidentally, the popes happened to have such observatories on hand. They called them churches. Poking a hole in the roof was a small price to pay for Easter date confidence. A handful of churches around Europe and Asia became solar observatories.
Fortunately for us, scientists are easily pulled off on tangents, and a few centuries of experimentation, trial and error was required to really nail down the Easter calculation. Matters of light diffraction and the distance from the center of the earth to the floor of the church had to be addressed. During this time Galileo and his ilk had stumbled onto a few interesting work products that the church was less than thrilled with.
The guy who finally mastered the Easter problem was Francesco Bianchini, who was allowed to build a meridian line diagonally across the floor of the giant church of Santa Maria degli Angeli in Rome. The church owed its size to the fact that it was actually built as a bath during the reign of Emperor Diocletian (284 – 305 AD) and was then converted to a church by Pope Pius IV in 1560 with the assistance of Michelangelo. Pius set about to avenge Diocletian’s Christian victims by converting a part of the huge pagan structure built “for the convenience and pleasure of idolaters by an impious tyrant” to “a temple of the virgin.”
Bianchini’s meridian is perhaps the major point of tourist interest within Santa Maria degli Angeli – science surrounded by faith. Charles Dickens once visited Italy and didn’t really care for it. He did marvel at one such solar observatory in a church, noting how the sun beams slowly and accurately marked out time among the kneeling people.