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Today's calendar
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Today almost everyone takes the precision of our calendar for granted, unaware of the long threads spooling out from our clocks and watches backward in time, running through virtually every major revolution in human sciences, all linked to the measurement of time.
Looking back reveals a struggle for our current calendar's acceptance and the story of Christopher Clavius, a sixteenth-century Jesuit who worked hard to create and defend it, only to have his name forgotten over those years that he helped to fix.
The fix was needed because of an error in the original calendar established by Julius Caesar in 45 bc, who mistimed his year so that it ran eleven minutes short--a deficit that accumulated gradually over the centuries. Eventually, the calendar fell back several days against the true astronomic year--a glitch first discovered three centuries before Clavius, when a sickly English friar named Roger Bacon dispatched a strident missive to Rome.
Addressed to Pope Clement IV, it was an urgent appeal to set right time itself. Calculating that the calendar was losing an entire day every 125 years, Roger Bacon informed the supreme pontiff that there was a surplus of time that over the centuries had accumulated to nine days. He declared that this drift, if left unchecked, would eventually shift March to the dead of winter and August to the spring.
However, it took another three centuries before the Church was willing to concede that its calendar was in error, when Pope Gregory XIII finally fixed the calendar in 1582. Gregory's reform came after he appointed a calendar commission in the early 1570s, led by the Bavarian mathematician Christopher Clavius, SJ (1537-1612), a quiet hero of the Gregorian correction.
Christopher Clavius helped revolutionize the calendar, yet little exists to flesh out who he really was. In a portrait of Clavius rendered in 1606 he is dressed in a simple Jesuit robe and a four-cornered hat. A portly, satisfied-looking man with a pudgy, bearded face, he looks sympathetic, even kind--the sort of scholar who is serious but never stuffy, smart but not precocious; one that students are fond of, and one that politicians and prelates feel comfortable assigning to commissions.
Born on March 25, 1537, in the Bavarian town of Bamberg, Clavius's life to us is a blank page until he joined the Society of Jesus in Rome on April 12, 1555. Studying in Rome and later at the University of Coimbra in Portugal, Clavius returned to Rome in the early 1560s to finish his education and then to teach at the Jesuits' own Collegio Romano, where he became a professor of mathematics. But for a few short trips, he stayed in Rome until his death.
To his contemporaries Clavius was a revered sage of math and astronomy, acclaimed as "the Euclid of his times" in part because he penned a widely used translation of the original Euclid, along with several works considered important in his day. Even the era's greatest scientific firebrand, Galileo Galilei, came to him for validation of his telescopic observations of the moon, sun, and planets. Clavius hailed them as important to astronomy, but since he was a confirmed defender of Ptolemy's geocentric system, he disagreed with Galileo's interpretation that craters on the moon, Venus passing through its phases, and moons around Jupiter suggested Copernicus was correct.
Clavius, a staunch defender of an earth-centered universe, was nonetheless flexible enough to constantly update his own theories to incorporate Copernican data and Galileo's observations, attempting to squeeze them into an increasingly strained Ptolemaic interpretation.
 Christopher Clavius, professor of mathematics at the Jesuits' Roman College (on left), was asked by Pope Gregory XIII to correct the Julian calendar; our Gregorian calendar is the result. |
Clavius's willingness after 1582 to at least consider new ideas as Rome's senior astronomer seems to have exercised a restraining influence on the inevitable showdown between the ideas of Copernicus and those of Ptolemy, primarily benefiting the young Galileo, whose reputation was enhanced by Clavius's support of his telescopic discoveries. Galileo forgave him for rejecting the Copernican theory, a shortcoming he blamed on the old man's age, and judged Clavius to be "worthy of immortal fame."
Yet Clavius today is nearly unknown. It was his bad luck to have lived between Copernicus (Clavius was five years old when De revolutionibus was published) and Galileo, who burst onto the scene in Clavius's final years.
But more than anything, Christopher Clavius is obscure because he adhered to a worldview that turned out to be wrong, making him a hero to traditionalists but a fool to those who came later.
Despite some of his views, Clavius's contributions to calendar reform cannot be overlooked. Clavius was surprisingly young when Pope Gregory named him to his new calendar commission, convened in the mid 1570s. Clavius was the man behind the scenes who shepherded the reform through the minefields of scientific controversy before and after 1582.
Many astronomers found fault with the new calendar, including several mathematicians in Prague who refused to help the bishop there revise the calendar of feasts because they claimed to find the science unsound. Others were angry at the pope's order to eliminate ten days from the year 1582, which Clavius's commission recommended to correct the drift in the calendar and to restore it to its original dates in Roman times. Clavius worked hard to defend the new calendar, ensuring that it would spread beyond the handful of countries that initially accepted it.
In 1595 he wrote a refutation of a Protestant astronomer named Michael Maestlin, directed at the calendar's many critics, called Novi calendarii Romani apologia, adversus Michaelem Maestlinum, (Defense of the new Roman calendar, in reply to Michael Maestlin). He explained, among other things, why the commission adopted a system of mean rather than absolute motions, a necessity given that the year runs an inconvenient 365 days 5 hours and 49 minutes. Clavius's commission took the leap year system inaugurated by Caesar (3 years of 365 days followed by a fourth year of 366 days), which creates a mean year of 365 1/4 days, and modified it to compensate for the eleven-minute error.
In a rather elegant and easily remembered reform, he and his colleagues invented the leap-century rule, which cancels the leap year when it falls on a century year, except in those century years divisible by four. In other words, 1700, 1800, and 1900 were not leap years, but the year 2000 is one--a formula that creates a mean year very close to the true year.
Clavius also defended the use of a mean by pointing out that it was impossible for all Christians to celebrate Easter at exactly the same moment, given the spread of Christianity across several meridians. In 1606 Clavius answered his critics in the 800-page Explicatio (Explanation). In all, Clavius penned six treatises on the calendar, characteristically well reasoned and scientifically sound documents that went a long way toward quieting the criticism and smoothing the way for reform in countries that initially hesitated to go along with the new calendar.
Some four centuries later, Clavius's calendar, commonly referred to as the Gregorian
calendar, is used by billions of people around the world who unknowingly pay homage to this
long-forgotten Jesuit who literally saved us all from falling out of time. 
David Ewing Duncan, author of Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year has written other books and numerous articles that have appeared in Atlantic Monthly, Smithsonian, and the New York Times, among other publications. Mr. Duncan has also been a commentator on National Public Radio's All Things Considered.