de Dondi's Astrarium
Hi-Tech, 14th Century style
Who?
Giovanni de Dondi, a physician, and son of a physician (Jacopo de Dondi). It was not unusual for a physician to be interested in the stars, but with Giovanni it was more like an obsession. It seems that his interest in astronomy and horology was inherited from Jacopo, since it was Jacopo who designed the astronomical clock in the Piazzi dei Signori, Padua (referred to in Astronomical clocks of the Middle Ages) - one of the first, if not the first of its type.
What?
He built an extraordinary astronomical clock, which he called an 'astrarium' or 'planetarium'. Fashioned out of brass, it was in effect a proto-computer. By means of complex mechanisms, fashioned out of brass, it calculated the positions of the planets, following the method perfected by Ptolemy in the 2nd century AD and recorded in his 'Almagest'.
Where?
The city of Padua, north-east Italy - about 30 miles from Venice, but inland. The university at Padua was started in 1222, making it the second oldest university in Italy. During the next 400 years it was an important centre of learning. The curriculum was based on the newly translated works of Aristotle, but with an emphasis on the scientific as opposed to the theological implications of those works. William Harvey (who discovered the circulation of the blood in the human body), Copernicus and Galileo all studied or taught at Padua. Jacopo de Dondi and his son moved from nearby Chioggia to Padua in 1248. Giovanni became Professor of Medicine at the university in 1352.
How long?
By this time (1352) Giovanni had already been working on his Astrarium for four years. It was to be another 12 years before his masterpiece was completed (1364). A friend stated that he did all the work himself.
How many parts?
There were 107 wheels and pinions in the Astrarium. The whole thing was worked by a movement of the verge and foliot type (picture - from article by John H.Lienhard No. 1535 Inventing the Clock). The results were displayed at two levels.
- At the upper level, seven dials showing the ecliptic longitude (i.e. position in the Zodiac) of Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon (in that order - the classic arrangement laid down by Ptolemy). The Sun dial was combined with an astrolabe-type dial (see Astronomical clocks of the Middle Ages) which showed the movement of the starry firmament, which is the reason that this dial is also known as the Primum Mobile dial. The motion of the backgound stars was always regarded as the 'primary motion' of the heavens, from which the movements of the sun, moon and planets were derived as 'secondary motions'.
- At the lower level, a 24-hour clock dial (equal hours), a dial showing the position of the nodes of the moon's orbit, an annual calendar wheel giving the date and saint(s) associated with that day, and finally an indicator giving the dates of the church's movable feasts for the current year: most importantly, Easter, but also Septuagesima, Quinquagesima, Rogation Sunday and Whitsun.
How do we know?
The original Astrarium was lost. In all probability it survived long enough for Leonardo da Vinci to see it. There exist drawings by Leonardo of the Mars and Venus dials - and Leonardo did spend a year or so at Pavia, near Milan, in about 1490, which is where de Dondi took the clock when he left Padua (1370's). In 1381 he presented it to his patron, the Duke Gian Galeazzo Visconti to keep in his library. By all accounts people came a long way to see the wonderful clock and marvel at it.
However, de Dondi wrote a detailed account of the design of the Astrarium, and how he made it. The manuscript was much copied (twelve copies are known today) - with the result that it has been possible in the 20th century to make several reconstructions of the Astrarium. There are examples in the
- Smithsonian Institution, Washington
- Museum of Science and Technology, Milan
- IBM headquarters, New York
- Theodore Beyer Collection, Zurich
- Science Museum, London
More recently, a half-size replica has been made by an Italian clock-maker, Carlo G. Croce. Visit his web-site to get a good idea of the appearance of the Astrarium.