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Europe, Circa 1775

BRASS UNIVERSAL EQUINOCTIAL RING DIAL

Unsigned. The outermost Meridian ring with a latitude scale suspends the dial by a sliding indicator which can be moved along the Meridian Ring and set with the clamp screw. The inner Equinoctial ring with Roman numerals rotates on and locks onto stops on the front and back. Central slotted indicator bar with a sliding pinhole gnomon, the sun indicator. Setting the date on the gnomon adjusts it to the sun's appropriate declination, allowing the shadow to indicate its altitude on the Meridian Ring in the same manner as on a mariner's astrolabe.
Diameter 6.5".
Condition: Excellent. No apparent signs of restoration or repair.

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  • Provenance:
    Christie's, South Kensington; Scientific Instruments, 1985.
    Purchased from the above by the The Kelton Collection of Marine Art & Artifacts.

    Self-orienting, this type of dial was very convenient to use at sea because a compass is not necessary, the method of suspension does not necessitate a steady base, it can be used in any latitude and it can be folded flat for storage. In favorable weather conditions this ring dial could be used at sea to obtain accurate readings 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.

    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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