A Slight Problem

As established by Julius Caesar, the solar year was reckoned as 365 days with an extra day added during a "leap year" every four years. In this way, the natural period of the seasonal year was understood to be 365 1/4 days in length. As explained by the Venerable Bede, "The Sun's year is complete when it returns to the same place with respect to the fixed stars after 365 days and 6 hours, that is a quarter of a whole day."

By this reckoning, the time from one Vernal Equinox to the next should have been 365 days, 6 hours. However, it was not known to either Julius Caesar or the Venerable Bede that in fact the solar year was precisely 365 days, 5 hours, 48 minutes and 42 seconds. As a result, the time from one Vernal Equinox to the next was overestimated by 11 minutes and 18 seconds.

We might be tempted to say that this is a tiny amount, what difference can 11 minutes make over the span of a year? Very little difference, but over a century of time, it can add up. This minor annual discrepancy amounts up to about three extra days every 400 years. And over many centuries, the accumulated days became considerable.

In the time of Caesar, the Vernal Equinox occurred on March 24. By the time of the Council of Nicaea, the Equinox was arriving on March 21. Throughout the Medieval period, the Equinox was arriving earlier and earlier. Consequently, the date of Easter was arriving earlier and earlier. Given enough centuries, Easter would regress backward through the calendar and begin to approach Christmas!

By the 1500s the Julian calendar had slipped 10 days from the Council of Nicaea, and the Equinox was arriving on March 11. Calendar reform became a topic for discussion among church authorities, in part because the nature of the problem was poorly understood. The revolutionary work of Mikolaj Kopernik (a.k.a. "Copernicus") was inspired by the effort to reform the calendar. 

In 1543 he wrote, "When the Lateran Council was considering the question of reforming the Ecclesiastical Calendar, no decision was reached, for the sole reason that the magnitude of the year and the months and the movements of Sun and Moon had not yet been measured with sufficient accuracy."

(Of course, Kopernik's solution included the novel idea of placing the Earth in orbit around the Sun, but that's another story.)

The final solution was adopted 40 years later, in 1582. Upon consulting with the learned astronomers of the time, Pope Gregory XIII issued a proclamation that restored the calendar: "So thus that the Vernal Equinox, which was fixed by the fathers of the Nicene Council at March 21, is replaced on this date, we prescribe and order that there is removed, from October of the year 1582, the ten days which go from the 5th through the 14th inclusively. The day which follows the 4th, when one traditionally celebrates St. Francis of Assisi, shall be the 15th."