Gregory and His Calendar

An article by CatholicTech Intern Dominic Andres

This week we commemorate the Papal Bull Inter Gravissimas, which, in 1582, established the Gregorian Calendar, named after Pope Gregory XIII, who issued the bull.

The history of this calendar begins more than three thousand years ago, in ancient Egypt. The greatness of the Egyptian Empire came from its location on the Nile river, which gave the Egyptians a strong agricultural economy, and with it an advanced culture. Since the Nile’s annual cycle of floods was key to this agricultural system, Ancient Egyptian Astronomers needed to develop an accurate calendar.

The most common kinds of calendar are Solar and Lunar. The Lunar Calendar is perhaps more intuitive, because the cycle of changes in the moon is directly visible. However, since the Earth’s position relative to the sun is the cause of the seasons, the most important annual cycle, the Solar Calendar is generally more useful. The Egyptians realized this, and so they developed one of the first Solar Calendars.

Now we jump forward a thousand years to the Roman Empire. The first Roman Calendars were Lunar, but in the year 45 BC, Emperor Julian proclaimed a new, Solar Calendar. Recognizing the improved accuracy of the Solar Calendar, and consulting the Egyptians, who were now vassals of Rome, he produced the Julian Calendar. This calendar assigned 365.25 days to each year, the most accurate measure they could produce at the time. This length was decided by measuring the time from the sun’s highest point at the solstice, or longest day of the year, from one year to another. They used gnomons, simple astronomical tools which consisted mainly of a stick in the ground, to find the correct point.

The Julian Calendar lasted for another fifteen hundred years. It was adopted by all citizens of the Roman empire, including the Catholic Church. Since Christianity has a regular cycle of feasts, accurate timekeeping is very important to the Church. Unfortunately, the Julian Calendar’s year of 365.25 days is actually incorrect! Because of this, Pope Gregory needed to institute a new Calendar, to keep the cycle of feasts in its proper place. They had gotten misaligned, falling in odd places in the Lunar cycle, which was used alongside the Solar Calendar when determining the placement of feasts.

Now, by the 1580s, science had advanced a great deal, and Gregory was able to have his astronomers calculate a more accurate year at 365.2425 years. At the same time, he declared that ten days that year would be skipped, putting the new year in the correct place, relative to the solstices, at the same time he established its proper length.

Pope Gregory did the world a great service when he used the resources of the Church to establish a better calendar. Accurate timekeeping is extremely important to science, culture, and religion, and the calendar is one of the most common ways of tracking time. Having a good calendar has contributed to everything from evolutionary theory to Christian worship. Let us thank Pope Gregory XIII for his great work, and keep him, and all the faithful departed, in our prayers.