Sidereal Angle Clock

This clock is highly unusual as it does not tell the time in hours minutes and seconds but instead by the Earth's angular rotation. The dial shows degrees, arc minutes and arc seconds.
The clock was originally purchased by Sir George Augustus William Shuckburgh (1751-1804) for use in his own observatory alongside a five foot equatorial telescope, made by Jesse Ramsden. Delivered and set up in October 1789 by John Arnold, the clock has the same function as Flamsteed’s angle clock, made by Thomas Tompion in 1691.

Elected as a fellow of the Royal society in 1774, Sir George wrote in the philosophical transactions of 1793 about this regulator

‘Whereas most astronomical clocks shew sidereal time in hours and minutes, which is afterwards, in the course of computation, reduced to degrees and minutes; this machine shews the degree and minute of the equator, that is upon the meridian at any given instant, directly without reduction. This is of considerable convenience in observations, out of the meridian, with an equatorial instrument’

Arnold’s bitter rival, Thomas Earnshaw, refers to the article as a ‘pompous publication’ in his appeal to the public and was impelled to point out that the timekeeper’s error of three seconds per day on its rate was not something to applaud.

Following Shuckburgh’s death the regulator and the Ramsden equatorial were presented to the Royal Observatory in 1811. The clock was stored but never put into use; though it suffered from corrosion through neglect it is one of few regulators in the collection to have remained unaltered. The clock was transferred in 1929 to the Science Museum and in 1967 returned on loan to the National Maritime Museum where it is currently displayed next to the Bradley transit circle.

The movement has substantial 5mm thick plates with chamfered corners, which are united by six stout baluster pillars secured by steel screws and domed brass washers. The train consists of five wheels driven by a large flat brass cased weight with single brass pulley and double line running from a barrel with eighteen turns, Harrison’s maintaining power and a solid greatwheel. The barrel arbor runs on two pairs of anti-friction wheels pivoted in large bridges which are mounted to the front and backplates. With the exception of the greatwheel all wheels have six straight crossings and wheels above the centre wheel are triple screwed to their brass collets. The jewelled dead-beat escapement has a lightly constructed steel escape wheel and steel pallets with ruby nibs.

The five-rod grid-iron pendulum has a brass faced bob with rectangular cut-outs as typically found on Arnold’s temperature compensated pendulums. The outer and central rods are steel and enclose two rods of brass colour, but possibly made from a combination of silver, brass and zinc (as described in Philosophical Transactions 1793). The pendulum runs against a silvered beat scale, reading 0 to 5 degrees.

The twelve inch silvered brass dial is mounted to the movement via four screwed dial feet and has a typical regulator layout but reads degrees, arc minutes and arc seconds of Earth rotation rather than hour minutes and seconds; the recessed dial below the signature is marked for tens of degrees and will make a full rotation corresponding with the Earth’s spin. The outer chapter marked one to ten shows single degrees of angle, the subsidiary dial shows ten arc minutes and tens of arc seconds.

The case, veneered in figured mahogany, is branded with the Royal Observatory’s mark, a broad arrow surmounted by an eight pointed star, to the hood case and trunk door. The flat topped hood has an ogee moulded cornice, glazed apertures to the top and sides with a finely moulded inset glazed front door. The trunk has a concave throat and base mouldings, straight sides and a moulded door set with two glazed apertures. Located on the lower inside corners of the door are two guide wheels to prevent the descending weight from catching against the inside of the case. The rectangular base has a raised moulded panel to the front section and a double moulded plinth.

Object Details

ID: ZAA0563
Collection: Timekeeping
Type: Clock
Display location: Not on display
Creator: John Arnold & Son
Date made: circa 1789
People: Science Museum Group; Royal Greenwich Observatory
Credit: Science Museum Group
Measurements: Overall: 1915 x 430 x 265 mm

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