Instruments · Astronomical chronometer
Astronomical chronometer no. 342 by George Margetts
- Maker / attribution
- George Margetts (maker)
- Date
- 1783
- Historical period
- Precision & Scientific Horology (1750 – 1850)
- Country of origin
- Unrecorded in source
- Object type
- Astronomical chronometer
- Dimensions
- 410 x 330 x 310 mm
- Holding institution
- Royal Museums Greenwich
- Accession / object no.
- ZBA1219
- Credit line
- National Maritime Museum, Greenwich, London. Acquired with the assistance of The Art Fund and Friends of the National Maritime Museum, 1998.
- Catalogue title (original)
- 342 (English)
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Record summary
Held and catalogued by Royal Museums Greenwich (object number ZBA1219), this is an astronomical chronometer by George Margetts, dated 1783. The Atlas places it in the Precision & Scientific Horology period (1750 – 1850) of its Instruments collection. Recorded dimensions: 410 x 330 x 310 mm. The record is illustrated with 6 photographs.
Assembled from the source record's structured fields. Dates and attributions are given as the source states them; where a field is missing the Atlas leaves it blank rather than estimating.
Catalogue description
Catalogue description — Royal Museums Greenwich
Astronomical chronometer. Thomas Earnshaw made this watch for George Margetts to Margett's design of complex astronomical dialling, but incorporating Earnshaw's spring detent escapement invention. These early chronometers by Earnshaw were usually marked with a stamp and serial number, to denote that a fee was paid to Thomas Wright, who funded Earnshaw's patent costs. There is a small clock dial in the centre of the main dial, which has a planispheric representation of the night sky for the latitude of London, moving in real time.
Historical context
Precision & Scientific Horology (1750 – 1850)
Harrison solves longitude; Mudge invents the lever; Breguet's Paris workshop defines the modern watch.
The longitude problem was solved by the marine chronometer and lunar distance methods; dividing engines made precision scales possible, and observatories set time by transit instruments.
Read the period in the timelineMore instruments from this periodEarlier: Early Modern Horology
Atlas type guide
Marine chronometers
A marine chronometer is a precision timekeeper carried at sea to find longitude: the navigator compares local noon, observed with a sextant, with the time at a reference meridian kept by the chronometer. Each hour of difference corresponds to fifteen degrees of longitude. John Harrison proved the method in the 1760s; Pierre Le Roy, Ferdinand Berthoud, John Arnold and Thomas Earnshaw then developed the detent escapement and the compensation balance that made chronometers practical to manufacture.
By the nineteenth century London, Liverpool, Le Locle and Glashütte supplied navies and merchant fleets. A typical box chronometer has a spring-detent escapement, a fusee to equalise mainspring torque, a bimetallic compensation balance with a helical balance spring, and is mounted in gimbals inside a wooden box so that it stays level.
What to look for
- Gimbals and a three-tier wooden box with a glazed inner lid.
- A power-reserve ('up and down') indicator on the dial, usually showing 56 hours on two-day instruments.
- Spring-detent escapement and a cylindrical (helical) balance spring visible through the movement.
- Admiralty or navy marks (broad arrow), and rating certificates from observatories.
- The maker's number on the dial and movement; many makers' registers survive, allowing dating.
Dating clues
- Earnshaw-type spring-detent chronometers became standard after about 1800.
- Auxiliary compensation devices on the balance appear from the mid-nineteenth century.
- Quartz and radio time signals ended routine use of mechanical chronometers in the 1970s and 1980s.
General guidance written by the Atlas editors about this kind of object. It is not a statement about this particular piece; for that, rely on the source record above and on physical examination.
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