Pope Gregory XIII and his calendar reform commission were mandated to calculate the proper date of Easter, thus reaffirming the import of Catholicism in a Western Europe challenged by Protestantism. The reformed calendar, adopted as a human construct and not as a necessary truth, also acknowledged the imperfection of human knowledge.

The calendar in common use during the Renaissance in Europe was noticeably out of harmony with the true length of both the tropical year (the time it takes the earth to complete the seasonal cycle) and the sidereal year (the time it takes the earth to return to the exact point with respect to a fixed star in its orbit). This calendar, instituted by Julius Caesar in 46, counted an average of 365.25 days per tropical year (365 plus 1 every fourth or leap year). Since the true tropical year is only 365.2422 days long—or 11 minutes, 12 seconds shorter than that projected by the Julian calendar—the Julian dates had been falling behind the those of the sun. Calendar, Julian
Calendar, Gregorian
Gregory XIII
Lilius, Aloysius
Clavius, Christopher
Copernicus, Nicolaus
Gregory XIII
Clavius, Christopher
Chacón, Pedro
Danti, Ignazio
Sirleto, Gugliemo
Lilius, Aloysius
Copernicus, Nicolaus

That Julius’s calendar was seriously flawed had been known for many centuries. Its deficiencies were of particular concern to the Christian Church, which depended on both the solar and lunar calendars for the celebration of Easter, Easter;determining date of[date] its principal ecclesiastical feast. At the Council of Nicaea in 325, Easter had been established as the first Sunday after the first full moon after the vernal equinox; the vernal equinox was then assumed to fall on March 21. With each passing century, the date of the vernal equinox moved up about three quarters of a day (falling on March 11 by 1582), whereas the lunar calendar was mismatched with respect to the moon’s real cycles by about four days in 1582. The simultaneous dependence on two faulty calendars caused the date of Easter to fall on inappropriate days (with respect to the Nicene Council’s original intentions) because of the large discrepancy between the conventional date of March 21 for the vernal equinox used for the calculations and the real vernal equinox. Easter;calendar reform and Earlier attempts at calendar reform had foundered because of a lack of consensus on the part of scientists and the lack of interest on the part of heads of state.

Pope Gregory XIII took up the cause as a consequence of the final session of the Council of Trent Trent, Council of (1545-1563) (1563), during which Pope Pius IV (1559-1565) had given the mandate to reform the breviary. In Gregory’s eyes, reforming the breviary implicitly required the revision of the solar and lunar calendars so that moveable feasts such as Easter could be calculated properly. Soon after his election, Gregory established a papal commission, composed mainly of clerics, to address the problem both scientifically and theologically. Important members included the Jesuit mathematician and astronomer, Christopher Clavius, and the sixteenth century Spanish historian, Pedro Chacón, both of whom authored important documents during and after the deliberations. Other members were the Dominican mathematician and cosmographer, Ignazio Danti (1537-1586), and scholar-cardinal, Gugliemo Sirleto (1514-1585), president of the commission, who was instrumental in pressing for a final consensus.

Aloysius Lilius’s proposal was accepted by the commission. His solution was simple and easy to implement, and it was relatively accurate. Although the original goal of the commission had been to devise an error-free calendar that reflected the motions of the true tropical year—as opposed to hypothetical figures based on mean values, as had always been done—this idealistic goal was eventually abandoned. The astronomical data then available was recognized as being too inaccurate and, furthermore, there was no universally accepted model of planetary motion within which to locate that data. Nicolaus Copernicus’s monumental book De revolutionibus orbium coelestium (1543; On the Revolutions of the Heavenly Spheres, 1952; better known as De revolutionibus
De revolutionibus (Copernicus) ), which theorized that the earth revolved around the sun, was mined for its updated astronomical data, but his theory of planetary motion was considered one of the least appealing of the many competing models of the universe then in circulation.

Lilius’s solution to the calendar problem sidestepped such existential questions and made no claims to total accuracy. By canceling the leap year on century years not divisible by 400, he eliminated three days every 400 years, resulting in an average year length of 365.2425. This calendar would take three thousand years to run a day ahead. With similar logic, the lunar calendar was to be reduced by one day every 300 years for seven such periods and then again one day was to be dropped after 400 years. These two reforms, applied to a year that began correctly, would guarantee an appropriate date for Easter for millennia.

Although Lilius died before the commission had made any official decisions, his manuscript served as the basis for Gregory’s reform after a shortened version written by Chacón (known as the “Compendium”) was sent to Catholic rulers for approval in 1578. Despite some concerns, criticisms, and counterproposals, the papal bull Inter Gravissimas was signed February 24, 1582. The new calendar was to go into effect that year. To compensate for the ten-day drift that had occurred since the time of the Council of Nicaea (325), Gregory suppressed ten days in the month of October so that October 15 followed October 4 in 1582. Some Catholic countries that abided by the reform chose to distribute the lost days in other ways.

The papal bull generated much controversy among both Catholics and Protestants. Clavius, commonly dubbed the “Euclid of the sixteenth century,” was charged by Gregory to defend the new calendar and its Easter reckoning, which he did in a number of publications. In 1603, he published a lengthy definitive explanation of the reform entitled Romani calendarii a Gregorio XIII P. M. restituti explicatio
Romani calendarii a Gregorio XIII P. M. restituti explicatio (Clavius) . By then, all Catholic and some Protestant nations were following the Gregorian calendar, whereas most Protestant nations did not. It was not until the mid-eighteenth century that all major European powers were on Gregorian time, England being among the last to convert in 1752. The Eastern Orthodox Church enacted its own calendar reform in 1923.

Pope Gregory XIII’s momentous reform of the calendar was less scientific triumph than affirmation of Catholic tradition. Enacted on the heels of the final decrees of the Council of Trent, the Gregorian calendar commission adopted as its point of departure the original theological principles for the calculation of Easter put forward by the Council of Nicaea in 325. Sponsored by the Emperor Constantine to resolve doctrinal differences among the various Christian communities, this first ecumenical council was the spiritual predecessor of the Council of Trent and the origin of a unified Catholic doctrine. Linking calendar reform to the Nicene Council’s original intentions thus amounted to reinforcing the historical relevance of the Catholic Church in a period when it was under intense pressure from Protestant factions.

The reform commission’s ecclesiastical motivation, moreover, allowed it to push for reform expeditiously. Following Lilius’s lead, the commission’s recognition of humanity’s imperfect ability to collect astronomical data and the incompleteness of its understanding of God’s universe was an important milestone in the reform process: It made it permissible to back a simple, easy-to-implement calendar adjustment that was self-consciously a human construct rather than a perfect map or true reflection of celestial motion. Nevertheless, the reform that was adopted was remarkably accurate, given that the data available had been collected without the benefit of a telescope.

• Archer, Peter. The Christian Calendar and the Gregorian Reform. New York: 1942. An overview of the theological and astronomical principles behind the calculation of the dates of Christian feast days, Easter in particular, and the effect this had on Gregory XIII’s plans for calendar reform.
• Coyne, G. V., M. A. Hoskin, and O. Pederson, eds. Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate Its Four-Hundredth Anniversary, 1582-1982. Vatican City: Specola Vaticana, 1983. A collection of essays on the scientific, technical, theological, and historical issues and outcomes surrounding Gregory’s calendar reform, published by the Vatican in commemoration of the event. The most important book on this subject to date.
• Duncan, David Ewig. Calendar: Humanities Epic Struggle to Determine a True and Accurate Year. New York: Avon Books, 1998. A lively and well-written general introduction to the calendar as a subject, with several chapters devoted to Gregory’s reform.
• Richards, E. G. Mapping Time: The Calendar and Its History. New York: Oxford University Press, 1998. A historical and comparative analysis of calendars used throughout the world, including a special section on the history of determining the date of Easter.
• Swerdlow, J. “The Origin of the Gregorian Civil Calendar.” Journal for the History of Astronomy 5 (1974). A succinct discussion of the issues facing the Gregorian calendar commission.

1462: Regiomontanus Completes the Epitome of Ptolemy’s Almagest

c. 1478-1519: Leonardo da Vinci Compiles His Notebooks

c. 1510: Invention of the Watch

1543: Copernicus Publishes De Revolutionibus

1545-1563: Council of Trent

1572-1574: Tycho Brahe Observes a Supernova

1580’s-1590’s: Galileo Conducts His Early Experiments

1600: William Gilbert Publishes De Magnete