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Perpetual calendar

Illustration from 1881 U.S. Patent 248872, for a perpetual calendar paperweight. The upper section is rotated to reveal one of seven lists of years (splitting leap years) for which the seven calendars below apply.
A 50-year "pocket calendar" that is adjusted by turning the dial to place the name of the month under the current year. One can then deduce the day of the week or the date.

A perpetual calendar is a calendar valid for many years, usually designed to look up the day of the week for a given date in the past or future.

For the Gregorian and Julian calendars, a perpetual calendar typically consists of one of three general variations:

  1. Fourteen 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 (the year that does not have a February 29) with each of the seven starting on a different day of the week, and seven for each leap year, again with each one starting on a different day of the week, totaling fourteen. (See Dominical letter for one common naming scheme for the 14 calendars.)
  2. 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.[1] The seven calendars may be combined into one, either with 13 columns of which only seven are revealed,[2][3] or with movable day-of-week names (as shown in the pocket perpetual calendar picture).
  3. A mixture of the above two variations - a one-year calendar in which the names of the months are fixed and the days of the week and dates are shown on movable pieces which can be swapped around as necessary.[4]

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 Nürnberger Handschrift 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 and Quinquagesima. This is the first known instance of a tabular form of perpetual calendar allowing the calculation of the moveable feasts that became popular during the 15th century.[5]

The chapel Cappella dei Mercanti, Turin contains a perpetual calendar machine made by Giovanni Plana using rotating drums.

  1. ^ U.S. patent 1,042,337, "Calendar (Fred P. Gorin)".
  2. ^ U.S. patent 248,872, "Calendar (Robert McCurdy)".
  3. ^ "Aluminum Perpetual Calendar". 17 September 2011.
  4. ^ Doerfler, Ronald W (29 August 2019). "A 2010 "graphical computing" calendar". Retrieved 30 August 2019.
  5. ^ 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.

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