Dennis the Small, a 6th century expert on canon law, has had an enduring, if unsung, influence on the affairs of the modern world. It was thanks to his calculations that the 2000th anniversary of Christ's birth was recently celebrated. Dionysus Exiguus, as he was then known, was charged in 525 by Pope John I with setting dates in the Christian calendar. Though he was a good mathematician, the accuracy of Dennis' calculations was limited by the information available at the time.
Determining the date of Christ's crucifixion--and therefore the date of Easter--presented an especially difficult challenge. Theologians had decreed that Easter should be marked on the Sunday after the first full moon after the vernal equinox, but without accurate solar and lunar calendars how could the church fathers predict the dates in the future? In The Sun in the Church (Harvard University Press, 366 pages) Professor J.L. Heilbron describes one inspired solution to working out the dates: metal lines inscribed in the floor of a large dark building with a hole in the roof to allow the noon sun to shine onto it.
According to tradition Christ died on the day of the vernal equinox, which in Roman times always fell on March 25. But the Julian calendar then in use was based on a solar year of 365.25 days rather than the more accurate 365.2422. Over the centuries those few minutes each year caused a cumulative inaccuracy in the dates, but without accurate astronomical instruments astronomers were unable to compute corrections.
Heilbron's book tells of the struggle to determine dates more accurately, including a little-known aspect of the history of the calendar--the use of churches as giant sundials to make astronomical measurements. The first of these meridians was built by Toscanelli in the Duomo of Florence around 1475. The cosmographer and mathematician Egnatio Danti put one into the Church of Santa Maria Novella 100 years later after using early navigational instruments to demonstrate to his patron, Cosimo di Medici, how the system would work. But the Santa Maria Novella line did not produce the accuracy Danti hoped for and it was left to Domenico Cassini 75 years later to achieve precision by altering another Danti meridian in the Church of San Petronio in Bologna.
As successive astronomers used the meridians they became aware that discrepancies showing up in their observations could not be accounted for in a system in which the sun revolved around the earth--clearly, the earth was rotating around the sun. Using churches to house meridians forced the Catholic hierarchy to reject a literal interpretation of the Bible's version of the earth's role in the universe and to accept the heliocentric principle. Thus a tiny hole in lofty church roofs not only shed light upon the mysteries of the calendar; it also illuminated the closed and darkened minds of church fathers.