Travel Through Time: The Gregorian Calendar

With the start of a new year and a new calendar, it is timely to reflect on how calendars have evolved and changed throughout the centuries. One volume in the Rare Book Collection has particular value for this endeavor, the Martyrologium Romanum ad novam kalendarii rationem et ecclesiasticae historiae veritatem restitutum.

The RBC edition of the Martyrologium Romanum was published in 1598, just fifteen years after Pope Gregory XIII introduced the Gregorian calendar.  Before 1582, most of the western world used the Julian calendar, introduced by Julius Caesar in 45 BCE.  The Julian calendar had 365 days divided into 12 months with a leap day added to February every 4 years, making each year exactly 365.25 days long.

Under the Julian system, the equinoxes and solstices advanced by 11 minutes annually with respect to the calendar.  While this seems like a small increment, this meant that by the 16th century, the spring equinox was falling on March 11th rather than March 21st.  This was a particular problem for the Catholic Church because the date of Easter each year depended in part on the full moon after the equinox, so the shift in the calendar caused Easter to be celebrated earlier and earlier in the year.  To solve this problem Gregory XIII instituted a small reform.  His Gregorian calendar moved ahead 10 days and would omit three leap years every four centuries.  The Gregorian calendar is still in use today, and while it remains out of sync with the astronomical calendar by twenty-six seconds, it will take thirty-five centuries before the calendar is off by an entire day.

The Martyrologium Romanum was published by the Typographia Apostolica Vaticana, the official printing press of the Vatican.  The 1589 edition is the third and was updated to include the feast days of Nereus and Achilleus (two early Roman martyrs) in the calendar. This combination of a calendar and theological history was not intended for a single year, but was intended for reuse each year.  To that end, rather than assign the day of the week to each date, every entry has what is known as a Dominical Letter. January 1 starts with A, and the sequence runs through G and then repeats. It would be announced each year on which letter all Sundays fell.

Epact table

Epact table

Because lunar dates were also valuable, particularly in determining the date of Easter as well as some other moveable feasts, each entry also lists the ages or phases of the moon in a table, depending on the epact (or age of the moon on January 1) of the year, which can be determined using charts at the beginning of the volume.  If we’ve done the calculations correctly, the age of the moon for 2015 will be listed under the character “k” for each entry in this volume and all Sundays fall under dates with the Dominical Letter “D.”

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