IT looks like a flying saucer, and to those viewing it on the monastery wall in Verona where it had hung since the Middle Ages, it can have had hardly less of an impact.
Catalogued as "almost certainly the largest and finest extant
example of a computistical instrument used for astronomical and
mathematical calculations", the Astronomical Calendar of San Zeno,
a 15th century ancestor of the analogue computer, was offered by
Sotheby's on July 6 as part of this summer's
modest selection of illuminated manuscripts and miniatures.
Comprising three pine discs connected by a central spigot and
overlain with vellum, it is painted to the outer edge of the
largest 4ft 2in (1.28m) diameter disc with calendar entries, then
moving inwards, with nine columns of figures in Arabic numerals and
12 large miniatures of the astrological signs.
On the central small disc is a painted night sky with an offset
aperture that reveals phases of the moon painted on the middle
This calendar of volvelle form was produced for the Benedictine
monastery of San Zeno in Verona and hung in the chapter house in
1455, the year in which the abbot also commissioned a monumental
altarpiece for the basilica of San Zeno from Andrea Mantegna. Some
200 years later this religious community was decimated by plague,
but in the mid 18th century, a local antiquarian recorded it as
still hanging on the wall of the chapter house in the now
In 1770 the monastery was suppressed, the basilica became a
parish church and the remaining buildings and their contents were
sold off. Looting and further destruction followed with the
Napoleonic Wars, but somehow it survived - albeit with major cracks
to the wooden discs, scratching, scuffing and fraying to the
vellum, and centuries of accumulated dirt and resultant
The volvelle, of which this is the largest surviving example -
perhaps the largest ever made - is a relative of the astrolabe and
the planisphere, but instead of measuring distances or angles, is
designed to cross-reference sets of data.
Volvelles have their origin in Arabic science and although
Western examples are known as early as the 13th century, they were
not commonly found until the advent of printing and the appearance
of Peter Apian's Astronomicum Caesarum in 1540, a
remarkable work which made great use of such volvelles or moveable
discs with pull-cords in its illustrations and diagrams.
The Padua-Verona region has produced a significant number of the
surviving examples, as in the 15th and 16th centuries the
universities of north eastern Italy were home to much early
scientific research. Georg von Peurbach lectured on astronomy at
Ferrara; his student, Johannes Muller, or Regiomontanus, taught in
Padua, and Galileo moved there in 1592 to lecture on geometry,
mechanics and astronomy.
The Sotheby's cataloguer points out that this great example
stands quite apart. The vast majority of surviving volvelles occur
in manuscripts that would have seen restricted use in private or
institutional libraries, but this one was made for public display
and "...must have caused as much excitement within the early
scientfic communities around Verona as the Prague astronomical
clock did in Bohemia".
Every emperor, pope and dignitary to visit Verona, it is
suggested, must have stood in the chapter house and gazed upon it,
and scores of mathematicians and astronomers, Galileo included,
would have travelled to Verona to study it and take notes.
Cracked and dirty, the vellum coverings rubbed and frayed, the
Astronomical Calendar of San Zeno was cautiously valued at
£40,000-60,000, but that estimate was swept aside as it moved on to
sell at £210,000.
By Ian McKay
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