De Dondi: Wikis

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Jacopo and Giovanni de'Dondi (father and son) were scholars active in 14th century Padua, Italy, and are remembered today as being pioneers in the art of clock design and construction. The Astrarium, designed and built by Giovanni de'Dondi over a period of 16 years, was a highly complex astronomical clock and planetarium, constructed only 60 or so years after the very first mechanical clocks had been built in Europe, and demonstrated an ambitious attempt to describe and model the solar system with mathematical precision and technological sophistication.

Giovanni de'Dondi's Astrarium. Tracing of an illustration originally from his 1364 clock treatise, Il Tractatus Astarii. It doesn't show the complex upper section with its many wheels, but just the weights, escapement, and main gear train.

Contents

The Dondi family

The father, Jacopo de'Dondi (1290 – 1359), was a scholar and doctor, and graduated from the University of Padua before becoming municipal physician at Chioggia. He returned to the University in 1342 and lectured at the school of medicine. He also supervised the construction of a large public clock with a dial, commissioned by Prince Ubertino of Carrara. He may also have contributed to its design. The clock was installed in the tower of the Palazzo Capetanato, Padua in 1344. There is some evidence that it indicated and struck the hours from 1 to 24, and also that it displayed the age and phase of the moon and the place of the sun in the zodiac. Both tower and clock were destroyed in 1390, when the Milanese stormed the palace.

Giovanni de'Dondi lived with his father from 1348 to 1359, and shared his father's interest in astronomy and clockmaking. In 1348 he began working on what he called his astrarium or planetarium. He described in detail the design and construction of this project, which was to occupy him until 1364. His manuscripts provided enough material for modern clockmakers to build reconstructions. In 1371 he served as ambassador to Venice, but after the conflict between Padua and Venice in 1372, joined the University of Pavia, and served as diplomat and scholar until his death in Milan in 1389.

The Astrarium

The astrarium was considered to be a marvel of its day. Giovanni Manzini of Pavia writes (in 1388) that it is a work "full of artifice, worked on and perfected by your hands and carved with a skill never attained by the expert hand of any craftsman. I conclude that there was never invented an artifice so excellent and marvelous and of such genius".

De'Dondi writes that he obtained the idea of an astrarium from the Theorica Planetarum of Giovanni Campano da Novara (Johannes Campanus), who describes the construction of the equatorium.

The astrarium was primarily a clockwork equatorium with astrolabe and calendar dials, and indicators for the sun, moon, and planets. It provided a continuous display of the major elements of the solar system and of the legal, religious, and civil calendars of the day. De'Dondi's intention was that it would help people's understanding of astronomical and astrological concepts. (Astrology was then considered a subject worthy of study by the intellectual elite and was taken reasonably seriously.)

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Description

The astrarium stood about 1 metre high, and consisted of a seven-sided brass or iron framework resting on 7 decorative paw-shaped feet. The lower section provided a 24 hour dial and a large calendar drum, showing the fixed feasts of the church, the movable feasts, and the position in the zodiac of the moon's ascending node. The upper section contained 7 dials, each about 30cm in diameter, showing the positional data for the Primum Mobile, Venus, Mercury, the moon, Saturn, Jupiter, and Mars.

De'Dondi constructed the clock, with its 107 gear wheels and pinions, entirely by hand. No screws were used, and every part was held together by over 300 tapering pins and wedges, with some parts being soldered. Most of the wheels have triangular shaped teeth, although some are blunt-nosed. In some cases, de'Dondi used near-elliptical wheels, in order to more accurately model the irregular motions of the planets (using the Ptolemaic epicycles rather than Kepler's ellipses). On some of these wheels, the teeth varied in size and spacing along the wheel's periphery. To indicate dimensions in his descriptions, de'Dondi used units such as the width of a goose quill, the thickness of a blade of a knife, or the breadth of a man's thumb.

For data on the motion of the planets, he consulted the Alfonsine tables, compiled in about 1272.

The clock movement

The clock movement had a balance wheel regulated to beat at the rate of 2 seconds. Its simple wheel train turned a dial marked on the margin with a scale of 24 equal hours and 10 minute intervals. The dial rotated anticlockwise against a fixed pointer, showing mean time, and could be adjusted if necessary by intervals of 10 minutes of time by sliding out a pinion of 12 teeth that meshed with the 144 teeth of the main dial. On each side of the clock dial was a fixed plate or 'tabula orientii', graduated with months and days of the Julian calendar for the purpose of determining the times of the rising and setting of the mean sun for the latitude of Padua (about 45°deg N). At the time the clock was made, the dates of the solstices were 13 June and 13 December (Old style or Julian Calendar).

The calendar wheel

The annual calendar wheel or drum in the lower section was about 40cm across. This drove the calendar of movable feasts, and the dials above. Around the outside of the wheel was a broad band divided into 365 strips, each containing numbers that indicated the length of daylight, the dominical letter, the name of the saint for that day, and the day of the month. Alternate months were gilded and silvered, and the engraved letters filled alternately with red and blue enamel. De'Dondi didn't introduce any indications or allowances for leap year - he recommended stopping the clock for the entire day.

The Primum Mobile dial

Directly above the 24 hour dial is the dial of the Primum Mobile, so called because it reproduces the diurnal motion of the stars and the annual motion of the sun against the background of stars. It is basically an astrolabe drawn using a south polar projection, with a fixed tablet and a rete of special design that rotated once in a sidereal day. The rete was provided with 365 teeth, but was driven by a wheel with 61 teeth which made 6 turns in 24 hours. Thus the rete rotated once in 365/366 of a mean solar day, which equated 366 successive meridian transits of the vernal equinox with 365 similar transits of the sun. De'Dondi realised that his approximations didn't correspond with the exact length of the solar year, and recommended stopping the clock occasionally so that it could be adjusted.

Planetary dials

Each of the 'planetary' dials used complex clockwork to produce reasonably accurate models of the planets' motion. These agreed reasonably well both with Ptolemaic theory and with observations. For example, de'Dondi's dial for Mercury uses a number of intermediate wheels, including: a wheel with 146 teeth, two oval wheels with 24 irregularly shaped teeth that meshed together, a wheel with 63 internal (facing inwards) teeth that meshed with a 20 tooth pinion. The 63-tooth wheel made one rotation a year, non-uniformly because of the oval driving wheels, and made the main indicator wheel rotate through 63/20 × 12 signs of the zodiac each year. This is equivalent to 37 signs and 24°, a good approximation to the value of 37 signs and 24° 43' 23" required by theory.

What happened to the clock?

In 1381 de'Dondi presented his clock to the Duke Gian Galeazzo Visconti, who installed it in the library of his castle in Pavia. It remained there until at least 1485. It may have been seen and drawn by Leonardo da Vinci. The clock's final fate is unknown.

Modern reconstructions

Because de'Dondi described most of the more complex components of his clock in his manuscripts in considerable detail, it has been possible for modern clockmakers to build convincing - if sometimes speculative - reconstructions. Seven of such reconstructions were built by Peter Haward of Thwaites and Reed, London,examples of these can be found in the Smithsonian Institution, The Time Museum Illinois and in the science museum in London.

References

  • Bedini and Madison, 1966 "Mechanical Universe : the Astrarium of Giovanni de'Dondi" (Transactions of American Philosophical Society)
  • King, Henry "Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks", University of Toronto Press, 1978
  • Baillie G. H., Lloyd H. A. and Ward F. A. B., "The Planetarium of Giovanni de Dondi citizen of Padua", 1974 London

See also

External links

  • a replica built by Italian clockmaker Carlo G Groce [1]

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