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A perpetual calendar is a calendar valid for many years, usually designed to allow the calculation of the day of the week for a given date in the future. For the Gregorian and Julian calendars, a perpetual calendar typically consists of one of two general variations: * 14 one-year calendars, plus a table to show which one-year calendar is to be used for any given year. These one-year calendars divide evenly into two sets of seven calendars: seven for each common year (year that does not have a February 29) that starts on each day of the week, and seven for each leap year that starts on each day of the week, totaling fourteen. (See Dominical letter for one common naming scheme for the 14 calendars.) * Seven (31-day) one-month calendars (or seven each of 28–31 day month lengths, for a total of 28) and one or more tables to show which calendar is used for any given month. Some perpetual calendars' tables slide against each other, so that aligning two scales with one another reveals the specific month calendar via a pointer or window mechanism.〔, "''Calendar (Fred P. Gorin)''".〕 The seven calendars may be combined into one, either with 13 columns of which only seven are revealed,〔, "''Calendar (Robert McCurdy)''".〕〔(【引用サイトリンク】title=Aluminum Perpetual Calendar )〕 or with movable day-of-week names (as shown in the pocket perpetual calendar picture. Note that such a perpetual calendar fails to indicate the dates of moveable feasts such as Easter, which are calculated based on a combination of events in the Tropical year and lunar cycles. These issues are dealt with in great detail in Computus. An early example of a perpetual calendar for practical use is found in the manuscript GNM 3227a. The calendar covers the period of 1390–1495 (on which grounds the manuscript is dated to c. 1389). For each year of this period, it lists the number of weeks between Christmas day and Quinquagesima. This is the first known instance of a tabular form of perpetual calendar allowing the calculation of the moveable feasts which became popular during the 15th century.〔Trude Ehlert, Rainer Leng, 'Frühe Koch- und Pulverrezepte aus der Nürnberger Handschrift GNM 3227a (um 1389)'; in: Medizin in Geschichte, Philologie und Ethnologie (2003), p. 291.〕 == Other uses of the term "perpetual calendar" == * Offices and retail establishments often display devices containing a set of elements to form all possible numbers from 1 through 31, as well as the names/abbreviations for the months and the days of the week, so as to show the current date for the convenience of people who might be signing and dating documents such as checks. Establishments that serve alcoholic beverages may use a variant that shows the current month and day, but subtracting the legal age of alcohol consumption in years, indicating the latest legal birth date for alcohol purchases. * Certain calendar reforms have been labeled perpetual calendars because their dates are fixed on the same weekdays every year. Examples are The World Calendar, the International Fixed Calendar and the Pax Calendar. Technically, these are perennial calendars. Their purpose, in part, is to eliminate the need for perpetual calendar tables, algorithms and computation devices. * In watchmaking, "perpetual calendar" describes a calendar mechanism that correctly displays the date on the watch 'perpetually', taking into account the different lengths of the months as well as leap days. The internal mechanism will move the dial to the next day.〔(【引用サイトリンク】title=Mechanism Of Perpetual Calendar Watch )〕 These meanings are beyond the scope of the remainder of this article. 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「Perpetual calendar」の詳細全文を読む スポンサード リンク
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