Atsuko Iwanami: The Struggle Over Calendar Reform—Europe from the Perspective of Calendar Calculation

The history of the calendar is the history of human intellectual endeavor.

Calendars tailored to human activities and purposes—such as region, faith, and lifestyle—have provided rhythm and tempo to our lives. In Europe, Julius Caesar (c. 102 BC – 44 BC) designated 46 BC as having 445 days, resolving the discrepancy between the calendar year and the tropical year (the time it takes for the sun to complete one cycle) in one fell swoop. Thereafter, the average year was set at 365.25 days with a leap year every four years. However, even after the implementation of this Julian calendar, the discrepancy with the tropical year was not completely resolved and continued to trouble people for a long time. This was because the tropical year, which is currently approximately 365.242189 days, was treated as 365.25 days.

A particularly contentious issue was determining the date of Easter, the most important movable feast in the Christian calendar. To ensure that the Sunday of Jesus' resurrection did not overlap with the Jewish Passover (based on a lunisolar calendar), the Council of Nicaea in 325 decreed that Easter would be the first Sunday after the full moon following the spring equinox. To accurately measure the date of Easter, which combines the solar and lunar calendars, the method of calendar calculation known as "computus" was devised. While the electronic computer was born in the 20th century, the "computus" had already been devised as a method of calendar calculation far back in medieval Europe.

It was difficult for scholars to reach a consensus on the calculation of the Easter date, and regional differences emerged depending on which method was adopted. Among the scholars who vied over the calculation of Easter was Dionysius Exiguus (c. 470 – c. 550), who was instrumental in introducing the era known today as "anno Domini" (AD).

The calendar dispute seemed to have finally reached a resolution with the Venerable Bede (c. 673–735), who laid the foundations of calendar law in the first half of the 8th century and was widely read throughout the Middle Ages. However, during the reign of Charlemagne, fierce debates once again broke out among scholars. Charlemagne was a ruler who made educational and cultural policies, known as the Carolingian Renaissance, a pillar of his administration. He focused on developing the educational infrastructure across his territories and, alongside linguistic proficiency, emphasized astronomy—essential for correct calendar calculation—and sought to embark on calendar reform.

Accurate observation of celestial movements is essential for calculating a calendar that visualizes time. Since medieval Europe was a Christian society, it would be premature to assume there was no cultural or academic exchange due to a confrontational structure with non-Christians. People in medieval Europe greedily accepted cutting-edge scientific knowledge, such as mathematics and astronomy necessary for calendar calculation, from the non-Christian Arabian world. At the end of the 10th century, Gerbert of Aurillac, who would later become Pope Sylvester II, studied in the Iberian Peninsula—which served as a window to Arabian scientific knowledge—and is said to have lectured at the cathedral school in Reims using the astrolabe and armillary sphere he mastered there. Furthermore, contemporaries believed that it was Pope Sylvester II who introduced the ancestral forms of Arabic numerals, which made calculations easier, to the European world. In the first half of the 12th century, he was condemned by William of Malmesbury as a man who had made a pact with the devil. While this story is reminiscent of Goethe's Faust, the 12th century saw intermittent Crusades on one hand, while the translation of natural scientific knowledge from Arabic was actively promoted on the other, leading to the flowering of the 12th-century Renaissance.

Discussions surrounding the calendar continued even 1,400 years after the implementation of the Julian calendar. A century before Copernicus proposed the heliocentric theory in "On the Revolutions of the Heavenly Spheres" (1543), Nicolaus Cusanus (1401–1464) wrote "On the Harmony of the Calendar" (1436), repeatedly pointing out the discrepancy between the Julian calendar year and the tropical year.

It was Pope Gregory XIII (1502–1585) who broke this deadlock. Aloysius Lilius, who received the order to calculate the optimal calendar, modified the existing leap year system. He corrected the discrepancy between the tropical year and the calendar year by decreeing that among years divisible by 100, only those whose century number is divisible by 4 (such as 1600 and 2000) would be leap years. This new calendar was implemented in October 1582, with the day following Thursday, October 4, designated as Friday, October 15. While maintaining the order of the days of the week, they attempted to remove the 10-day discrepancy at once. However, regional time differences occurred upon the introduction of this Gregorian calendar. It was first accepted by Catholic countries such as Italy, Spain, and Portugal; France followed later in December of the same year, followed by the Netherlands. Protestant regions were much later; while Regensburg, a Catholic region, introduced it the following year in 1583, it was not until 1699 that it was finally introduced in Nuremberg.

Britain, which still follows its own path today, was also hesitant to introduce the new calendar. It was finally introduced in Britain in September 1752 after careful preparation, but the day following September 2 was made September 14, leading to demonstrations over the "lost 11 days." In Japan, the calendar reform was implemented on January 1, 1873. Even now, the Greek Orthodox Church uses the Julian calendar or the Revised Julian calendar rather than the Gregorian calendar. The history of the struggle over calendar reform, which seeks to understand nature based on precise measurement, will never end.