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Dondi’s astrarium, the eighth wonder of the world
History & Masterpieces

Dondi’s astrarium, the eighth wonder of the world

Thursday, 23 October 2008
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Christophe Roulet
Editor-in-chief, HH Journal

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6 min read

The Italian astronomer Giovanni da Dondi devised and built the first planetary clock, which he completed in 1364. Considered a pure product of Italian genius, it nonetheless finished on the scrap heap… though not without firing the imagination of the medievalists who set about its reconstruction.

Any visitor to the Musée International de l’Horlogerie in La Chaux-de-Fonds, the Paris Observatory, even the Leonardo da Vinci Museum of Science and Technology in Milan, the Smithsonian Institution in Washington, the London Science Museum or the Musée Beyer in Zurich will have contemplated Dondi’s astrarium… though certainly not with the same emotion as the contemporaries of its creator, Giovanni da Dondi (1318-1389). And for a simple reason: they will only have seen reproductions of the original which, in its day, was considered to be one of the finest examples of human genius. No less than the eighth wonder of the world. Its complexity was such that, following da Dondi’s death, it never really functioned correctly and, like any other broken clock, ended its days on the scrap heap.

“This in no way diminished Giovanni da Dondi’s glory,” write Gerhard Dohrn-van Rossum, Philippe Braunstein and Olivier Mannoni in their book History of the Hour. “On the contrary: because no-one was able to repair the mechanism or maintain it in working order, the maker of this construction, considered one of the wonders of the world in the Lower Middle Ages, was seen as an extraordinary genius to whom many of his contemporaries would pay tribute, among them Petrarch and Philippe de Mézières.”

A rule on the planet dials shows the actual position of the planet in the zodiac, i.e. the sign and degree where it is* © MIH
A rule on the planet dials shows the actual position of the planet in the zodiac, i.e. the sign and degree where it is* © MIH
Reconstructing the astrarium

“Let us not linger on the technical obstacles that prevented the astrarium’s proper functioning and instead consider the collaboration between a man of science, capable of describing in detail that which he implements, and the technicians capable of making, in metal, the toothed wheels that caused the seven sides of the tower to rotate. A society that can give form to this coinciding of theory and practical knowledge, and to such a degree of technical refinement, has at its disposal the productive resources that herald industrial society,” continues Philippe Braunstein in his work Travail et Entreprise au Moyen Âge. In other words, the astrarium, an exceptional object, is considered the first planetary clock, intended to function as an mechanised equitorium [[An astronomical instrument invented by Campanus of Novara in the 13th century and used to calculate the position of the planets by means of mobile discs, arranged as a physical representation of Ptolemy’s geometric constructions, without the help of calculation tables.]] in which a clockwork mechanism reproduces the movement of the planets.

Based on the works of Ptolemy, set out in Almagest [[Ptolemy decomposed the irregular movements of the planets as several regular movements. These were expressed in a table that could be used to calculate the position of the planets at any point in the year.]] some fifteen centuries before the heliocentric theory was accepted, the astrarium would have taken 16 years to research and develop before being finally completed and presented in 1364 (some exegetes say 1380). The political climate of that time forced Giovanni da Dondi to leave Padua, taking his masterpiece with him. Thus da Dondi’s birthplace, where his father had already distinguished himself as the maker of a public astronomical clock, would not fully bask in the glory of its university’s illustrious professor. In Pavia (Lombardy), where he took refuge, he received the visit of dignitaries, all dazzled by such ingeniousness and seemingly in no way disappointed to behold an object which all too often refused to function. “Alas, written records of the astrarium’s time in Pavia refer only to the repeated, and fruitless, attempts to restore it to working order. Its guardians must have grown weary of their thankless task as, after the early 16th century, all trace of the astrarium is lost,” explains Emmanuel Poulle in an article published in Pour la Science. Fortunately for posterity, when building his astrarium, da Dondi scrupulously recorded every stage in its making. Thanks to this document, the only one of its kind to have come to us from the Middle Ages, every detail of its construction is known.

The movement has a driving weight, verge escapement and fleury annular foliot* © MIH
The movement has a driving weight, verge escapement and fleury annular foliot* © MIH
A polygonal tower

Emmanuel Poulle describes the clock thus: “The astrarium is a seven-sided polygonal tower which stands around one metre high. The lower section contains the clock mechanism, in the centre, with a clock on one of the sides. This is an ordinary clock except that its dial is divided into 24 parts corresponding to the hours and that it is the dial which rotates around a fixed marker. The upper section contains seven dials, one on each side, which carry the mechanism for the seven planets: the Sun, the Moon and the planets known at that time, which are Mercury, Venus, Mars, Jupiter and Saturn. Each mechanism is independent of the others. The astrarium is intended as a reflection of the world. Since Ptolemy, it was accepted that the daily movement of the heavens drove the other celestial movements just as the clock mechanism commands the planetary mechanisms.”

Without going into technical detail – whereby each planet moves in a circle or epicycle which itself moves along a larger, usually eccentric, circle or deferent, the circular motion of the deferent being uniform when seen from the centre of the equant – several groups set about reproducing an astrarium according to medieval manuscripts, of which three different versions had been conserved. These texts were written in the vernacular, or a “disconcerting Latin” to borrow the paleographers’ expression, which made it all the more difficult to interpret the instructions they contained. They gave rise to different reproductions of the astrarium, the most noticeable differences being the calculations used for the gears and the elliptical contours of the wheels. According to Emmanuel Poulle, the only faithful reconstruction is that of the Paris Observatory, the previous two failing to follow da Dondi’s instructions to the letter. He concludes that “while the astrarium was inspired in scientific terms, this reconstruction shows that it no doubt never functioned correctly. The admiration shown by its visitors in the 15th century was less for what they saw than for what they had hoped or dreamed of seeing.”

*Replica of Giovanni da Dondi’s astrarium, presented to the Musée International de l’Horlogerie (MIH) and made according to da Dondi’s instructions by the Milanese watchmaker Luigi Pippa.
Height: 110 cm. Diameter: 90 cm.
Replica made in 1985.

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