The Gregorian calendar was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar. The Julian calendar assumed incorrectly that the average solar year is exactly 365. 25 days long. The Gregorian reform shortened the average year by 0. 0075 days to stop the drift of the calendar with respect to the equinoxes. In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter.
About Gregorian calendar in brief
The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar. The calendar spaces leap years to make its average year 365. 2425 days long. The Julian calendar assumed incorrectly that the average solar year is exactly 365. 25 days long, an overestimate of a little under one day per century. The Gregorian reform shortened the average year by 0. 0075 days to stop the drift of the calendar with respect to the equinoxes. In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter. The reform was adopted initially by the Catholic countries of Europe and their overseas possessions. Over the next three centuries, the Protestant and Eastern Orthodox countries also moved to what they called the Improved calendar, with Greece being the last European country to adopt the calendar in 1923. During the 20th century, most non-Western countries also adopted the calendar, at least for civil purposes. A leap year normally occurred every 4 years, and the leap day was historically inserted by doubling 24 February. However, it is now customary to number the days of February sequentially with no gaps, and 29 February is typically considered the leap year. Gregorian years are identified by consecutive year numbers. A calendar date is fully specified by the year, the month, and the day of the month. Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar.
Calendar cycles repeat completely every 400 years, which equals 146,097 days. Of these 400 years,. 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is 365+97400 days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. The year in both calendars consist of 365 day, with a leap day being added to February in the leap years. The only difference is that there is one day less in leap month every four hundred years, with the leap month of February being added in February every four 100 years. The Julian algorithm had caused the calendar to drift such that the spring equinix was occurring well before its nominal 21 March date. This date was important to the Christian churches because it is fundamental to the calculation of the date of Easter. It had been introduced by the early Church and thus introduced into the reality of the year in which it was celebrated. The motivation for the adjustment was to bring the celebration of Easter to the time of year of which it is celebrated. It has been described as ‘the most important calendar reform of all time’ It was instituted by papal bull Inter gravimas dated 24 February 1982, and is named after the pope, Gregory XIII, after whom the calendar is named. It is a solar calendar with 12 months of 28–31 days each.