The leap year is a well-known quirk in the Gregorian calendar by which much of the world organises itself.
What may be less well known is the part played by the Church’s Easter celebrations in the development of the leap year.
Professor Darius von Güttner is Dean of the Canberra campus of Australian Catholic University (ACU) and a specialist in church history. He shares with us that while leap years have ancient origins, it was the way they affected Easter that prompted Pope Gregory XIII in the 16th century to reform the widely used Julian Calendar.
Giving some background on the purpose and importance of leap years, Prof von Güttner explains that they are ‘an essential aspect of our modern calendar system’ and ‘serve a critical function in aligning our calendar year with the astronomical year, ensuring that seasonal and astronomical events occur at approximately the same time each year’.
The reason they are termed ‘leap years’ is because, following the additional day at the end of February, the days ‘leap’ forward. For example, in 2023, 1 March was on a Wednesday. In 2024, 1 March falls on a Friday rather than a Thursday.
The concept of a leap year dates all the way back to the ancient Roman calendar, reformed by Julius Caesar in 45 BC, Prof von Güttner says. The reason Caesar reformed the calendar then was because their lunar calendar was significantly out of sync with the astronomical seasons.
The Julian calendar introduced by Caesar was a solar calendar that ‘added an extra day every four years to account for the fact that an astronomical year is slightly longer than 365 days—about 365.25 days,’ he says.
In 1582, the new “Gregorian” calendar was introduced, refining the leap year rule to more accurately reflect the length of the solar year.
But it wasn’t a perfect calendar. ‘The addition of a leap day every four years introduced a small overcorrection because the actual solar year is approximately 11 minutes shorter than 365.25 days. By the 16th century, the cumulative effect of this overcorrection had become apparent, with significant religious implications.’
In AD 325, the Council of Nicaea decreed that the celebration of Easter should be on the first Sunday following the full moon after the spring equinox. Because of the Julian calendar’s overcorrection, the celebration of Easter had ‘drifted’ significantly from Nicaea’s decree.
‘To address this, Pope Gregory XIII convened a commission of experts to reform the calendar. In 1582, the new “Gregorian” calendar was introduced, refining the leap year rule to more accurately reflect the length of the solar year,’ Prof von Güttner explains.
How does the reformed Gregorian calendar work?
‘According to the Gregorian system, a year is a leap year if it is divisible by four, except for end-of-century years, which must be divisible by 400 to be leap years. This adjustment corrected the drift and realigned the calendar with the astronomical seasons, ensuring that the date of Easter remained consistent with the Council of Nicaea’s determination in AD 325.’
This story highlights just how important the relationship between religion, astronomy and mathematics has been to the development of our history and culture.
‘The need to accurately calculate the date of Easter—a fundamental aspect of Christian liturgy—was a major driver behind the Gregorian reform,’ Prof von Güttner points out. ‘This concern for maintaining the liturgical calendar reflects the broader role of the Church in mediating the relationship between human society and the cosmos.’
‘The connection between leap years and the history of the Church thus lies not only in the technical adjustments made to the calendar but also in the broader context of the Church’s role in shaping Western intellectual and cultural developments.’
As we mark leap years, we commemorate not just an additional day on our calendars but a profound legacy of historical insight, ecclesiastical authority and the continuous human endeavour to organise our lives in harmony with the cosmos.
Interestingly, too, the reform of the Gregorian calendar served an important purpose in what is referred to as the Counter-Reformation—the Catholic response to the 16th-century Protestant Reformation.
Through this reform, the Church sought not just to renew its spiritual authority, but also ‘to reassert its relevance and authority by addressing a practical problem that affected the daily lives of people across Christendom,’ Prof von Güttner says.
Ultimately, the marking of a leap year can remind us of the ways that domains that are not often thought of as related, like science and religion, have actually worked together for the betterment of humanity.
‘The introduction of the Gregorian calendar under Pope Gregory XIII corrected centuries of accumulated error, realigning human measuring of time with the solar year. This reform addressed practical concerns related to agriculture and religious observances. It demonstrated the interplay between science and faith in shaping human understanding of time.’
‘As we mark leap years,’ he says, ‘we commemorate not just an additional day on our calendars but a profound legacy of historical insight, ecclesiastical authority and the continuous human endeavour to organise our lives in harmony with the cosmos.’
