The Astronomical Calendar of San Zeno, with paintings of astrological signs, illuminated manuscript on vellum laid down on wood [north east Italy (Verona), mid fifteenth century (almost certainly 1455)]

Almost certainly the largest and finest extant example of an early computistical instrument used for astronomical and mathematical calculations, and an early ancestor of the analogue computer

provenance

Written and illuminated for the Benedictine monastery of St. Zeno in Verona, almost certainly in 1455, where it hung in the chapter-house (cf. Biancolini, Dei Vescovi e Governatori di Verone, 1757, p.21). The abbey was founded in the ninth century by Charlemagne's second son Pepin (773-810), and commanded immense wealth throughout the Middle Ages, acting 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 (c.1431-1506) for the basilica of St. Zeno. The Calendar in the outermost ring includes numerous commemorations of the rare saint, St. Zeno (including his deposition on 12 April, his translation on 21 May, his ordinatio on 8 December and the dedication of the basilica there to him on 10 December; cf. Avena and Callegari 'Un Calendario Ecclesiastico Veronese del Secolo XV^o, Madonna Verona xi, 1917). The community 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 being sold off. Subsequently, the buildings were looted and destroyed during the Napoleonic Wars.

text

This manuscript is a volvelle (a device with a number of rotating circles which allows the viewer to chart and calculate relationships between cyclical events). They are relatives of the astrolabe (which is used to tell time, measure height and distance or determine the angles of celestial bodies) and the planisphere (used in the calculation of the positions of the stars), but instead of measuring distances or angles, volvelles are designed to cross-reference sets of data, and are thus the ultimate ancestors of analogue-computers. They have their origin in Arabic science (particularly the work of the Persian Astronomer, Abu Rayhan Biruni, 973-1048), and Western examples can be found as early as the thirteenth century (Cambridge, Corpus Christi Coll.16 and 26). The devices were mentioned by Chaucer in his Treatise on the Astrolabe, but they were not common until the advent of printing and the publication of Peter Apian's Astronomicum Caesareum in 1540. They were the subject of an exhibition at the Grolier Club in 2004: Volvelles: the Magnificent Art of Circular Charting.

Interestingly, the area of Padua-Verona has produced a significant number of the surviving examples of this type of early scientific instrument. In the fifteenth and sixteenth centuries, the universities of north-eastern Italy were hotbeds of early scientific research. Georg von Peurbach (1423-61), often named the father of mathematical and observational astronomy in the West, lectured on astronomy in Ferrara; his student Johannes Müller or Regiomontanus (1436-76) lectured in Padua; and the physicist, mathematician, astronomer, and 'father of modern science', Galileo Galilei (1564-1642) moved to Padua in 1592 to lecture on geometry, mechanics, and astronomy.

The present manuscript stands quite apart. It is, to the best of our knowledge, the largest and finest example of this type of instrument to survive, and perhaps also to have ever been made. The vast majority of surviving examples are in manuscripts for private study, kept away from public view. The present example, on the contrary, was made for display, and must have caused as much excitement within the early scientific communities around Verona as the Prague Astronomical clock did in Bohemia. Every emperor, pope and dignitary to visit Verona must have stood in the chapterhouse and gazed upon it, and scores of mathematicians and astronomers, including Galileo himself, must have travelled to Verona to study it and take notes.

Working in from the outer edge, the manuscript here contains on its main disc (a) the date of the month, (b) the dominical letter, (c) the applicable feast and saints' days, (d) 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), (e) degrees of each sign of the zodiac, (f) 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), (g) the half length of the night in hours and then in minutes, (h) the length of the night in hours and then minutes, (i) the period in hours and then minutes elapsed between sunset and noon the following day, (j) miniatures of the signs of the zodiac with labels noting their nature, either benevolent or maleficent; and on its smaller disc (k) the scales of the age of the Moon; and through the hole in the central disc (l) the phase of the moon.