The Gregorian Reform: The Adjustment of Time

The Imperceptible Drift of the Julian Calendar

The Julian calendar, in its simplicity, represented a major step forward. However, it was based on an approximation: that a solar year lasted exactly 365.25 days. With the actual duration being about 365.2422 days, the Julian calendar was actually too long by a little over 11 minutes per year. While this difference seems tiny, it produced a considerable drift over the centuries.

By the 16th century, this accumulated discrepancy reached about 10 days. The spring equinox, an astronomical pillar supposed to fall around March 21, was now occurring around March 11 in the calendar. This shift posed a major theological problem for the Church, as the date of Easter, which depends on the equinox, was drifting further and further away from the celestial event it was meant to celebrate.

The Correction by Pope Gregory XIII

Faced with this astronomical evidence, Pope Gregory XIII enacted a bold reform in 1582 to resynchronize civil time with solar time. The correction was carried out in two steps:

• The Leap in Time: To instantly catch up, it was decreed that the day after Thursday, October 4, 1582, would be Friday, October 15, 1582. Ten days were thus "removed" from history to realign the calendar with the seasons.
• A New Rule for Leap Years: To prevent the drift from recurring in the future, the Julian rule was refined. A year is a leap year every 4 years, except if it is a century year (like 1700, 1800, 1900...) not divisible by 400. This exception removes 3 leap days every 400 years, providing a much more accurate approximation of the true solar year.

A Universal Calendar?

The Gregorian calendar, due to its accuracy, is now the international civil standard. However, its adoption was neither instantaneous nor universal. Catholic countries adopted it quickly, but Protestant and Orthodox nations took centuries to follow, creating a period of confusion where multiple dating systems coexisted in Europe. It is this calendar that sets the rhythm of our modern world, a sophisticated human construct, yet one that remains, like its predecessors, an attempt to frame the uninterrupted flow of celestial cycles.