Computus clock

The determination of the Easter date requires calculating astronomical and calendar cycles – the annual motion of the Sun round the celestial sphere, the evolution of the phases of the Moon, the cycle of the days of the week, particularities of calendars and some agreements like the date of the so-called ecclesiastical equinox.,[1] designated as March 21, irrespective of the actual astronomical observation by the Church of Alexandria in the beginning of the 4th Century.

Specific astronomical data which may be incorrect, misinterpreted or location dependent, were eliminated from the Easter date calculation by the invention of special paschal functions – “letters” and “numbers”.

The computations after the Gregorian reform of 1582 should also take into account additional corrections necessary due to particularities of the Gregorian calendar, notably the solar equation (taking into account some non-leap century years) and the lunar equation (for correction of the Metonic cycle)[3] The term “computus” as the description of the Easter date computation was proposed in 725 by the English Benedictine monk Bede in his treatise “De temporum ratione” (“The Reckoning of Time”).

[4] Alexandrian computus, based on rules established by the Church of Alexandria, was universally used from the beginning of the 8th century until the Gregorian calendar reform of 1582.

The indication of the date of the upcoming Easter is one of the rarest astronomical functions of mechanical clocks and watches due to the high levels of complexity involved.

The clock has not survived, but the design and construction were described in detail by Dondi in his manuscripts[6] and provided enough material for modern clockmakers to build reconstructions.

A tabular computus with indications of the dominical letter, the epact, the 28-year solar cycle, the golden number and the indiction was made by the French clockmaker Auguste-Lucien Vérité in his monumental astronomical pendulum clock of Besançon Cathedral (France).

The computus module has 6 ecclesiastical functions – the 28-year solar cycle, the Gregorian epact, the Gregorian dominical letter, the golden number, the indiction and the indication of the day of the week of January 1 of the next year, so the computation of the Easter date may be provided with the help of the ecclesiastical functions even for the years after 1964, when the original Easter date disc is no more valid, as noted during restoration works in 2011–2012.

The most complicated type of computus clock is rather a kind of mechanical computer making automatic Easter calculations based on ecclesiastical indications at the beginning of a given year.

[15] At the start of each year, the computus changes the indications of the dates of Easter and moveable feasts on the annual calendar ring of the main dial in the central lower part of the clock.

The French clockmaker Daniel Marius Vachey [fr] also took his inspiration from the works of Jean-Baptiste Schwilgué and his astronomical clock of Strasbourg Cathedral.

The clock is also equipped with a dial for 6 moveable feasts and a perpetual calendar with date, day of the week, month and bissextile year indicators.

An extended program cam wheel mechanism was used in the display of the Easter date in the monumental astronomical clock of Beauvais Cathedral (France), built by the French clockmaker Auguste-Lucien Vérité from 1865 to 1868.

The cam wheel is calculated for a period of 300 years,[22] and the computus also has dials for indications of the dominical letter, the epact, the 28-year solar cycle, the golden number and the indiction.