18TH CENTURY BRASS UNIVERSAL EQUINOCTIAL RING DIAL European,circa 1775 Brass. Diameter 6.5".
18TH CENTURY BRASS UNIVERSAL EQUINOCTIAL RING DIAL
Unsigned. The outermost Meridian ring with a latitude scale. The dial is suspended from the Meridian Ring by a sliding indicator which can be moved along the Meridian Ring and set with the clamp screw. The inner (Equinoctial) ring has an hour scale engraved in Roman numerals I - XII on each half making a full 24 hours. This Equinoctial Ring rotates on and locks onto stops on the front and back. The slotted indicator bar at the center has a sliding pinhole gnomon (the sun indicator). The gnomon is slid to the index setting for the appropriate date, which adjusts it to the sun's declination for that date. The shadow of the sun will indicate it's altitude on the Meridian Ring the same as on a mariner's astrolabe.
Brass. Diameter 6.5".
Condition: Excellent. No apparent signs of restoration or repair.
Provenance: Christie's, South Kensington; Scientific Instruments, 1985. Purchased from the above by the The Kelton Collection of Marine Art & Artifacts. This type of dial was very convenient to use at sea because it is self-orienting in the meridian so that no compass is necessary, the method of suspension does not necessitate a steady base, and it can be used in any latitude. In favorable weather conditions this ring dial could be used at sea to obtain readings with an accuracy of within ten minutes. Although the precise origin of this instrument is unknown, its invention in the early 17th century was attributed to William Oughtred (1575-1660) by Henry Wynn in a pamphlet published in 1682. It remained in use into the mid-19th century. It was convenient to carry since it could be folded flat for storage. When the instrument is suspended and turned to allow the sun's rays to pass through the pinhole gnomon, the time is indicated on the hour scale of the equinoctial ring by a spot of light (except at midday when the sun is obscured by the meridian ring). In addition to telling time, the Universal Ring Dial can be used to determine the sun's declination.
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