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The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely used civil calendar.〔(Introduction to Calendars ). United States Naval Observatory. Retrieved 15 January 2009.〕〔(Calendars ) by L. E. Doggett. Section 2.〕〔The international standard for the representation of dates and times, ISO 8601, uses the Gregorian calendar. Section 3.2.1.〕 It is named for Pope Gregory XIII, who introduced it in 1582. The calendar was a refinement to the Julian calendar〔See Wikisource English translation of the (Latin) 1582 papal bull ''Inter gravissimas''.〕 amounting to a 0.002% correction in the length of the year. The motivation for the reform was to bring the date for the celebration of Easter to the time of the year in which it was celebrated when it was introduced by the early Church. Because the celebration of Easter was tied to the spring equinox, the Roman Catholic Church considered the steady drift in the date of Easter caused by the year being slightly too long to be undesirable. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923. The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory XIII's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by Aloysius Lilius.〔Moyer (1983).〕 His proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform. The Gregorian reform modified the Julian calendar's scheme of leap years as follows: 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.〔(Introduction to Calendars ). (15 May 2013). United States Naval Observatory.〕 In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. The canonical Easter tables were devised at the end of the third century, when the vernal equinox fell either on 20 March or 21 March depending on the year's position in the leap year cycle. As the rule was that the full moon preceding Easter was not to precede the equinox the equinox was fixed at 21 March for computational purposes and the earliest date for Easter was fixed at 22 March. The Gregorian calendar reproduced these conditions by removing ten days.〔Ziggelaar (1983), p. 223.〕 To unambiguously specify the date, dual dating or Old Style (O.S.) and New Style (N.S.) are sometimes used with dates. Dual dating uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates. Old Style and New Style (N.S.) indicate either whether the start of the Julian year has been adjusted to start on 1 January (N.S.) even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.) rather than the Gregorian (N.S.). The Gregorian calendar continued to use the previous calendar era (year-numbering system), which counts years from the traditional date of the nativity (''Anno Domini''), originally calculated in the 6th century by Dionysius Exiguus.〔(Nineteen-Year Cycle of Dionysius ). Introduction and first ラテン語:''argumentum''.〕 This year-numbering system, also known as Dionysian era or Common Era, is the predominant international standard today.〔The first known occurrence of ''Common Era'' in English dates to 1708. Years before the beginning of the era are abbreviated in English as either BC for "Before Christ", or as BCE for "Before the Common Era" Two era names occur within the bull ''Inter gravissimas'' itself, ラテン語:''anno Incarnationis dominicæ'' ("in the year of the Incarnation of the Lord") for the year it was signed, and ラテン語:''anno à Nativitate Domini nostri Jesu Christi'' ("in the year from the Nativity of our Lord Jesus Christ") for the year it was printed. (Les canons of Les textes fondateurs du calendrier grégorien ) 〕 ==Description== The Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days, but as in the Julian calendar, in a leap year, a leap day is added to February. In the Julian calendar a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years. In the Julian calendar, this leap day was inserted by doubling 24 February, and the Gregorian reform did not change the date of the leap day. In the modern period, it has become customary to number the days from the beginning of the month, and February 29th is often considered as the leap day. Some churches, notably the Roman Catholic Church, delay February festivals after the 23rd by one day in leap years.〔Richards, p. 101〕 Gregorian years are identified by consecutive year numbers.〔Clause 3.2.1 ISO 8601〕 The cycles repeat completely every 146,097 days, which equals 400 years.〔The cycle described applies to the solar, or civil, calendar. If the ecclesiastical lunar rules are also considered, the lunisolar Easter ''computus'' cycle repeats only after 5,700,000 years of 2,081,882,250 days in 70,499,183 lunar months, based on an assumed mean lunar month of 29 days 12 hours 44 minutes 2 seconds. (Seidelmann (1992), p. 582) (properly function as an Easter ''computus'', this lunisolar cycle must have the same mean year as the Gregorian solar cycle, and indeed that is exactly the case. )〕〔The extreme length of the Gregorian Easter ''computus'' is due to its being the product of the 19-year Metonic cycle, the thirty different possible values of the epact, and the least common multiple (10,000) of the 400 - year and 2,500 - year solar and lunar correction cycles. (Walker 1945, 218)〕 Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A calendar mean year is 365 days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds.〔The same result is obtained by summing the fractional parts implied by the rule: 〕 A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1). 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 (see the "beginning of the year" section below). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gregorian calendar」の詳細全文を読む スポンサード リンク
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