The Transition to the Gregorian Calendar


Myself . By T.V. Antony Raj


Do you know that people who lived in the year 1752 AD lost 11 days of their lives?

Normally, except in a few instances, a reform can bring about happier times in our lives. The Gregorian calendar, also called the Western calendar and the Christian calendar that we use today, is internationally the most widely accepted and used civil calendar. It was introduced by Pope Gregory XIII in 1582, by papal bull Inter gravissimas dated February 24, 1582 as a reform to the Julian calendar.

Calendar for September 1752

Look carefully at the image of the above calendar for September 1752. You will notice that 11 days, 3rd to 13th, are missing.

To better understand the calendar change of 1752 that led up to this particular occurrence of missing dates we must know a little about the history of major calendars starting with the Roman calendar.

The Roman calendar

The Roman calendar changed its form several times from the founding of Rome to the decline and fall of the Roman Empire. The original Roman calendar is believed to have been a lunar calendar, which may have been based on one of the Greek lunar calendars. As there are about 12 lunations (synodic months) in a solar year, this period (354.37 days) is sometimes called a lunar year with an average length of the synodic month of 29.530589 days. This requires the length of a month to be alternately 29 and 30 days (termed respectively hollow and full). At some point of history dates of months ceased to be connected with the lunar phases, but it is not known when it happened.

A common purely lunar calendar is the Islamic calendar or Hijri Qamari calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to 12 days. It comes back to the position it had relative to the solar year approximately every 33 Islamic years. It is used mainly for religious purposes, and in Saudi Arabia it is the official calendar. In other systems, a lunar calendar may include extra months added that synchronize it with the solar calendar.

The Julian calendar

Following the advice of Sosigenes, the Alexandrian astronomer, Julius Caesar established a more regulated civil calendar based entirely on the Earth’s revolutions around the sun. It took effect in 45 BC (709 AUC). It had three common years containing 365 days, and one year (leap year) containing 366 days (every fourth year). This twelve-month calendar, based on a solar (tropical) year, served for many years in perpetual cycle. This calendar is known as the Julian or Old Style (O.S.) and was used throughout the Roman Empire and by various Christian churches. It was the predominant calendar in most of Europe, and in European settlements in the Americas and elsewhere, until it was superseded by the Gregorian calendar.

The Gregorian Calendar

Pope Gregory XIII celebrating the introduction of the Gregorian calendar. Detail of the pope's tomb by Camillo Rusconi.

Early Christians of Jewish origin celebrated Easter, the resurrection of Jesus, as a new facet of the Jewish Passover festival as both are linked with much of their symbolism, In many languages, the words for “Easter” and “Passover” are etymologically related.

The early Roman Catholic Church found the steady drift in the date of Easter undesirable since it was tied to the spring equinox. In 325 AD, the First Council of Nicaea established the date of Easter as the first Sunday after the full moon (the Paschal Full Moon) after the March equinox. Therefore, the date of Easter varies between March 22 and April 25. A canon of the council specified that all Christians should celebrate Easter on the same day.

The Gregorian reform contained two parts:

  • a reform of the Julian calendar as used before Pope Gregory XIII’s time, and
  • a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter.

The reform was a modification of a proposal made by Aloysius Lilius, an Italian doctor, astronomer, philosopher and chronologist. His proposal included reducing the number of leap years in four centuries from 100 to 97, by making three out of four centurial years common instead of leap years. This part of the proposal had been suggested before by, among others, Pietro Pitati, an Italian astronomer and mathematician.

The Gregorian reform modified the Julian calendar’s scheme of leap years as follows:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

The Gregorian reform also took note of the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year and dealt with the accumulated difference between these lengths.

Between AD 325 (when the First Council of Nicaea was held, and the vernal equinox occurred approximately March 21st), and the time of Pope Gregory’s bull in 1582, the vernal equinox had moved backward in the calendar, until it was occurring on about March 11th, ten days earlier. Therefore, the Gregorian calendar began by skipping ten calendar days, to restore March 21st as the date of the vernal equinox.

The bull Inter gravissimas issued by Pope Gregory XIII on February 24, 1582 became the law of the Catholic Church in 1582, but it was not recognized by Protestant Churches, Orthodox Churches, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. It took almost five centuries before almost all Christians achieved that goal. The last European country to adopt the reform was Greece in 1923.