The Gregorian calendar - the basis of the modern 'western' calendar, the internationally accepted standard for ordinary life - was a modification of the Julian calendar to fit it better to the natural cycle of the year. It is named for Pope Gregory XIII, who introduced it in the Roman Catholic church (of which he was the head)in October 1582; it is still, as the standard modern 'civil calendar', the foundation of modern diaries and so on (although the Muslim year is still reckoned by a lunar calendar). The Gregorian developments were designed for two purposes.
- It corrected a comparatively small error in the Julian Calendar, which gave an average length of year as 365.25 days (365 days 6 hours), as opposed to the real length of an astronomical year, calculated by wikipedia as 365.24219 days (Ridpath, 2012 gives the the 'sidereal year' [one revolution of the earth on its orbit, measured against the 'fixed' stars] as 365.256 36 days, while agreeing with wikipedia on the length of the 'tropical year', [the distance between one mean equinox and the next]). The Gregorian calendar corrected the Julian calendar's length of year to 365.2425 days (365 days 5 hours 49 minutes 12 seconds) by removing three leap years in every four centuries, by laying down that "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400". The Gregorian calendar gives an average year of 365.2425 mean solar days, which amounts to an error of about one day every 3,300 years .
- The second correction. now largely of historical interest, was top bring the new calendar into conformity with the early church's calculation of the vernal equinox, and thus the date of Easter, which is connected to the Jewish feast of Passover, and calculated with reference to lunar months. (Thus it is variable.) Over the millennium, the days of calendar had drifted by ten from their proper place. So Gregory XIII's second reform was to 'lose', or remove, ten days.
- When Protestant Britain belatedly adopted the Pope's proposal (in 1752; without acknowledgement), it is said to have sparked 'Calendar Riots' with mobs shouting "Give us back our eleven days!" This appears to be an embellishment of an incidental squib in a satirical painting and later engraving: William Hogarth's An Election Entertainment, satirizing the corruption of the 1754 election in Oxfordshire. One of the issues raised by the Tories was popular dissatisfaction with the new calendar (one of the whig candidates, Viscount Parker, was the son of the Earl of Macclesfield, the astronomer who had chaired the Parliamentary committee leading to it; Hogarth shows a placard with the slogan "Give us back our eleven days". (The election, deeply corrupt, was eventually decided in favour of the Whigs, who both, in a double-member constituency, had fewer votes than their opposing Tories, by a vote in the House of Commons.)