Astrarium of Giovanni Dondi dall'Orologio
[1]: 8 [2]: 2:364–5 In the introduction, Dondi writes that his machine was built in accordance with the 13th-century Theorica planetarum of Campano di Novara, and to demonstrate the validity of the descriptions of the motion of heavenly bodies of Aristotle and Avicenna.
[3] The autograph manuscript was published in 1987 in a critical edition with colour facsimile and French translation by Poulle as the first volume of the Opera omnia of Jacopo and Giovanni Dondi.
Dondi writes that he obtained the idea of an astrarium from the Theorica Planetarum of Giovanni Campano da Novara, who describes the construction of the equatorium.
)[citation needed] 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 upper section contained 7 dials, each about 30 cm in diameter, showing the positional data for the Primum Mobile, Venus, Mercury, the moon, Saturn, Jupiter, and Mars.
In some cases, Dondi used near-elliptical wheels, in order to more accurately model the irregular motions of the planets (using the Ptolemaic epicycles rather than the ellipses as later worked out by Johannes Kepler).
To indicate dimensions in his descriptions, 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.
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 Padova (about 45°deg N).
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.
[citation needed] In 1381 Dondi presented his clock to Gian Galeazzo Visconti, duke of Milan, who installed it in the library of his castle in Pavia.
Because 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.
