Perpetual Calendar

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 of 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.)
Common year starting on: Mon Tue Wed Thu Fri Sat Sun
Leap year starting on: Mon Tue Wed Thu Fri Sat Sun
  • 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.

The seven calendars may be combined into one, either with 13 columns of which only seven are revealed, 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.

Read more about Perpetual Calendar:  Other Uses of The Term "perpetual Calendar", Perpetual Calendar Algorithms, Perpetual Julian and Gregorian Calendar Table

Famous quotes containing the words perpetual and/or calendar:

    Television knows no night. It is perpetual day. TV embodies our fear of the dark, of night, of the other side of things.
    Jean Baudrillard (b. 1929)

    To divide one’s life by years is of course to tumble into a trap set by our own arithmetic. The calendar consents to carry on its dull wall-existence by the arbitrary timetables we have drawn up in consultation with those permanent commuters, Earth and Sun. But we, unlike trees, need grow no annual rings.
    Clifton Fadiman (b. 1904)