Gregorian calendar

The Gregorian calendar is the calendar widely used in the Western world. A modification of the Julian calendar, it was first proposed by the Neapolitan doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 (Note: The papal bull Inter gravissimas was signed in the year 1581 for unknown reasons, but printed on 1 March in 1582. Although 1581 is often attributed to the use of years beginning on 25 March by the papacy, other contemporaneous papal bulls have years that do not agree with March years, let alone years since a pope was named or other types of years.)

Accuracy

The Gregorian calendar improves the approximation made by the Julian calendar by skipping 3 Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long, which has an error of about 1 day per 3300 years with respect to the mean tropical year of 365.2422 days but less than half this error with respect to the vernal equinox year of 365.2424 days. Both are substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days).

Related Topics:
Julian calendar - Mean solar day - Day - Year - Mean tropical year - Vernal equinox year

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

On timescales of thousands of years, the Gregorian calendar falls behind the seasons drastically because the slowing down of the Earth's rotation makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration. The equinox will occur earlier than now by a number of days approximately equal to 2. This is a problem that the Gregorian calendar shares with any rule-based calendar.

Related Topics:
Tidal acceleration - Leap second

~ ~ ~ ~ ~ ~ ~ ~ ~ ~