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Intercalation is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months. == Solar calendars == The solar or tropical year does not have a whole number of days (it is about 365.24 days), but a calendar year must have a whole number of days. The most common way to reconcile the two is to vary the number of days in the calendar year. In solar calendars, this is done by adding to a common year of 365 days, an extra day ("leap day" or "intercalary day") about every four years, causing a leap year to have 366 days (Julian, Gregorian and Indian national calendars). The Decree of Canopus, which was issued by the pharaoh Ptolemy III, Euergetes of Ancient Egypt in 239 BC, decreed a solar leap day system; an Egyptian leap year was not adopted until 25 BC, when the Roman Emperor Augustus successfully instituted a reformed Alexandrian calendar. In the Julian calendar, as well as in the Gregorian calendar, which improved upon it, intercalation is done by adding an extra day to February in each leap year. In the Julian calendar this was done every four years. In the Gregorian, years divisible by 100 but not 400, were exempted in order to improve accuracy. Thus, 2000 was a leap year; 1700, 1800, and 1900 were not. Epagomenal〔From ἐπαγόμενος, ''epagomenos'' (present participle passive of ἐπάγειν, ''epagein'' "to bring in") + -al〕 days are days within a solar calendar that are outside any regular month. Usually five epagomenal days are included within every year (Egyptian, Coptic, Ethiopian, Maya Haab and French Republican calendars), but a sixth epagomenal day is intercalated every four years in some (Coptic, Ethiopian and French Republican calendars). The Bahá'í calendar includes enough epagomenal days (usually 4 or 5) before the last month (`Alá’) to ensure that the following year starts on the vernal equinox. They are known as the "Festival of Ayyám-i-Há." 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「Intercalation (timekeeping)」の詳細全文を読む スポンサード リンク
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