1582 — Inter gravissimas
Papal bull Inter gravissimas, Gregory XIII, 24 February 1582
A dated document with a stated purpose: restore the vernal equinox to the place assumed by the Paschal tables, and stop the drift recurring.
The two operations
First, a one-off correction: ten days were removed, so that Thursday 4 October 1582 was followed directly by Friday 15 October 1582 in the states that adopted immediately. Second, a permanent rule change: century years are leap years only when divisible by 400, so 1600 and 2000 are leap years while 1700, 1800 and 1900 are not.
The stated reason
The bull's concern is the celebration of Pascha. The Julian drift had moved the equinox to around 11 March, while the Paschal computation assumed 21 March. The reform restored the equinox to 21 March and adjusted the lunar tables (the epacts) alongside the solar rule.
Who built it
The scheme is credited principally to the physician and astronomer Aloysius Lilius (Luigi Lilio), whose proposal was refined and defended by the Jesuit mathematician Christopher Clavius. It followed decades of discussion, including at the Council of Trent, which had asked for the calendar and liturgical books to be corrected.
The weekday was preserved
Worth stating plainly against a common claim: the reform deleted calendar dates, not days of the week. Thursday was followed by Friday. The seven-day cycle running back through the biblical period was not interrupted.
Precision achieved
Mean Gregorian year 365.2425 days against a tropical year of about 365.2422 — an error of roughly one day in 3,200 years, compared with the Julian error of one day in about 128 years. On the astronomy alone, the correction is not seriously disputed by anyone, East or West.
Citations on this file
Sources
- Inter gravissimas (1582) — Latin text with English translationFull text of the bull with commentary on the epact and leap-year provisions.https://www.fourmilab.ch/documents/calendar/ ↗
- U.S. Naval Observatory — calendar referenceGregorian leap rule, mean year length and the October 1582 date deletion.https://aa.usno.navy.mil/faq/calendars ↗
Links are pointers for further reading. The claim is what is sourced, not the website.