The Astronomical Calendar of San Zeno
San Zeno, Monastery of
Date of Creation:
The calendar of San Zeno is the largest and nest extant example of an early computational instrument used for astronomical and mathematical calculations, and an early ancestor of the analogue computer: in essence a monumental volvelle, a device with a number of rotating discs which allows the viewer to chart and calculate relationships between cyclical events. is astronomical calendar can show: the date of the month; the dominical letter; the applicable feast and saints’ days; the golden number (ie. the position of the year in the 19-year cycle, as devised by the ancient astronomer Meton of Athens, and within which for one hour on the particular day the moon returns to the same position relative to the earth which it held 19 years earlier); degrees of each sign of the zodiac; letters and numbers of the solar cycle (the revolution of 28 years in which the days of the month return to the same days of the week); the half length of the night in hours and then in minutes; the length of the night in hours and then minutes; the period in hours and then minutes elapsed between sunset and noon the following day; minia- tures of the signs of the zodiac with labels noting their nature, either benevolent or male cent; and on its smaller disc, the scales of the age of the moon; and through the hole in the central disc, the phase of the moon.
The calendar was created, written and illuminated for the Benedictine monastery of San Zeno in Verona, almost certainly in 1455, where it hung in the chapter-house. Founded in the ninth century by Charlemagne’s second son Pepin (773-810), the abbey commanded immense wealth throughout the Middle Ages, and acted as a significant patron to the arts in the region. At the same time as the present calendar was produced, the abbot, Gregorio Correr, commissioned a monumental altarpiece from the painter Andrea Mantegna (c1431-1506) for the basilica of San Zeno.
The calendar includes numerous commemorations of Zeno, amongst them his deposition (12th April), his translation (21st May), his ordination (8th December) and the dedication of the basilica to him (10th December), all shown in the outermost ring (cf. Avena and Callegari ‘Un Calendario Ecclesiastico Veronese del Secolo XVo’, Madonna Verona xi, 1917).
The community at San Zeno was decimated by the plague of 1660, and this calendar was recorded as still hanging on the wall of the chapterhouse in the near-deserted monastery by the local antiquarian Giovanni Battista Giuseppe Biancolini in the mid-eighteenth century. The monastery was finally suppressed in
1770, the basilica becoming a parish church and the remaining buildings and their contents were dispersed. Subsequently, the buildings were looted and destroyed during the Napoleonic Wars.
While sequestered with the monks in the monastery at San Zeno, the astronomical calendar was made to be used and for display. It must have caused as much excitement within the early scientific communities around Verona as the Prague astronomical clock did in Bohemia. It is easy to imagine every emperor, pope and dignitary that visited Verona standing in the chapterhouse and gazing upon it; mathematicians and astronomers, including Galileo himself, must have travelled to Verona to study it and take notes.
The closest, and only known relatives of this extraordinary calendar, in size and complexity of information and function are the famous astronomical clock in Prague, most of which originally dates from 1490, and heavily restored in more recent years; and the similar astronomical clock in St Mary’s, Gdansk, dating from the late-1460s, which was severely damaged in 1945 and subsequently heavily restored (cf. Biancolini, Dei Vescovi e Governatori di Verone, 1757, p.21).