Francois Ducommun-dit-Boudry, La Chaux de Fonds, 92 mm, circa 1825
A rare equinoctial sunring dial with original box and original hand-written operating instruction by Francois Ducommun
Meridian-ring : brass, suspension ring is attached to bracket by means of a hook and moves in grooved rim, engraved degree scale divided clockwise from 0°-90°, signature. Equatorial ring: brass, engraved, observe side with 24 hour division with inlaid Roman numerals. Bridge: one side has an engraved scale with zodiacal symbols, the other side with engraved Italian months, pinhole gnomon has indexes that move over the bridge scales.
Francois Ducommun (1763-1839)
Ducommun was a famous maker and designer of ring sundials, planetaria and planetary clocks who lived in La Chaux de Fonds. He is said to have built all his timepieces himself with the assistance of only a few selected craftsmen.
The equinoctial ring
The equinoctial ring dial is the mariner’s watch and an integral part of all navigation inventories. It can be used to determine time for all lines of latitude as well as for solving various navigational problems.
The ring dial was invented in the first half of the 17th century and soon became very popular because it was robust and folding, which make it easy to transport. In this case the usual hand aligned to the pole is replaced by a rectangular bar with a central slit; this is mounted diagonally in the vertically suspended ring. The inclination of the slider corresponds to the position of the celestial pole, so that when the ring is in the meridian, the bar is parallel to the Earth’s axis. A perforated slider sits in the central slit and is adjusted along a scale according to the height of the sun at the time of the measurement. A second ring that is vertical to the first ring and thus parallel to the equatorial plane has the hour markings. When the instrument is set, a sunray falls through the slit in the slider onto the inner ring and shows the time. Vice versa the fact that the light falls on the inner ring proves that the instrument is adjusted correctly, thus eliminating the need of a compass.
Source: "Uhren- und Messinstrumente des 15. bis 19. Jahrhunderts" by Samuel Guye / Henri Michel, p. 251.
"Nautische Antiquitaeten" by Jean Randier, Bielefeld 1973, p. 103.