The Gregorian calendar reform

By the sixteenth century, the equinox fell earlier in the Julian calendar than the position used in church calculations. Easter depended on conventions involving the equinox and lunar tables, bringing the accumulated difference into religious and administrative debate. Pope Gregory XIII introduced a reform in 1582, drawing on proposals associated with Luigi Lilio. Christopher Clavius explained its operation. Printed calendars and official instructions communicated the revised sequence of dates.

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In the first adopting regions, 4 October 1582 was followed by the date 15 October. Days and weekdays continued normally; their calendar labels changed. The revised rule keeps years divisible by four as leap years, except that century years must also be divisible by four hundred. Thus 1600 and 2000 are leap years, while 1700, 1800 and 1900 are not. Ninety-seven leap years in four centuries give an average year of 365.2425 days.

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Catholic states adopted the reform first, while other governments changed civil dates at different times. Britain and its colonies changed in 1752, and Russia in 1918. Taxation, wages, contracts and records required administrative coordination. As correspondence and trade expanded, a shared civil date system helped organize transport, meetings and accounts. Religious observance and local traditions could retain other calendars alongside the increasingly widespread Gregorian year.

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