– Introduction to Gregorian Calendar:
– Introduced in October 1582 by Pope Gregory XIII.
– Replaced the Julian calendar.
– Aims to make the average calendar year 365.2425 days long.
– Rule for leap years: divisible by four, except for centurial years not divisible by 400.
– Used in most parts of the world.
– Evolution from Julian to Gregorian Calendar:
– Julian calendar had an overestimate of the solar year.
– Gregorian calendar adjusted leap year calculations.
– Implemented to correct inaccuracies in the Julian calendar.
– Transitioned in October 1582.
– United States Naval Observatory played a role in the transition.
– Leap Year Calculation in the Gregorian Calendar:
– Leap years occur every four years.
– Exception for centurial years not divisible by 400.
– Example: 1700, 1800, 1900 are not leap years; 2000 is a leap year.
– Rule ensures the average calendar year is more accurate.
– United States Naval Observatory contributed to establishing the rule.
– Purpose of Implementing the Gregorian Calendar:
– Corrected the overestimation of the solar year in the Julian calendar.
– Average solar year is approximately 365.2422 days long.
– Julian calendar had a leap year every four years without exception.
– Gregorian calendar aimed for greater accuracy in year length.
– Implemented to align the calendar with astronomical phenomena.
– Impact of the Gregorian Calendar Adoption:
– Widely adopted across the globe.
– Improved accuracy in measuring time.
– Established a more precise calculation for leap years.
– Transition from the Julian calendar to the Gregorian calendar was significant.
– United States Naval Observatory played a role in the adoption process.
The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.
The rule for leap years is:
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. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.
There were two reasons to establish the Gregorian calendar. First, 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, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes. Second, in the years since the First Council of Nicaea in AD 325, the excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the March equinox 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. To reinstate the association, the reform advanced the date by 10 days: Thursday 4 October 1582 was followed by Friday 15 October 1582. In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter, because astronomical new moons were occurring four days before the calculated dates. Whilst the reform introduced minor changes, the calendar continued to be fundamentally based on the same geocentric theory as its predecessor.
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 gradually moved to what they called the "Improved calendar", with Greece being the last European country to adopt the calendar (for civil use only) in 1923. However, many Orthodox churches continue to use the Julian calendar for religious rites and the dating of major feasts. To unambiguously specify a date during the transition period (in contemporary documents or in history texts), both notations were given, tagged as 'Old Style' or 'New Style' as appropriate. During the 20th century, most non-Western countries also adopted the calendar, at least for civil purposes.
