The Julian and Gregorian Calendars

Excerpt from an article

by Peter Meyer


2. The Gregorian Reform

The average length of a year in the Julian Calendar is 365.25 days (one additional day being added every four years). This is significantly different from the "real" length of the solar year. However, there is uncertainty among astronomers as to what the length of the solar year really is (see Simon Cassidy's Erro in Statement of Tropical Year.). The main competing values seem to be the "mean tropical year" of 365.2422 days ("mean solar days") and the "vernal equinox year" of 365.2424 days. The difference of the length of the Julian calendar year from the length of the real solar year is thus 0.0078 days (11.23 minutes) in the former case and 0.0076 days (10.94 minutes) in the latter case.

Inter Gravissimas Whatever, this error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day. Thus as the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons. This was especially troubling to the Roman Catholic Church because it affected the determination of the date of Easter, which, by the 16th Century, was well on the way to slipping into Summer.

Pope Paul III recruited several astronomers, principally the Jesuit Christopher Clavius (1537-1612), to come up with a solution. They built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform. (The full text may be read in both the Latin original and a French translation by Rudolphe Audette, and in  an English translation, recently done by Bill Spencer.)

The Gregorian reform consisted of the following:


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Adoration of the Shepherds


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