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The Encyclopedia of Horology

Precision & navigation

Precision Timekeeping

Explore marine chronometers, longitude methods and historical rate records through cited guides, exact collection photographs and an angular time-error learning tool.

Read the evidence

Long-form illustrated guideLongitude and marine timekeeping: clocks, watches and lunar distancesUnderstand why reference time matters at sea, how Harrison’s instruments differed, and why astronomical navigation and chronometers developed together rather than in a simple winner-takes-all contest.Long-form illustrated guideReading chronometer rate records: error, correction and uncertaintyA documentary guide to interpreting time comparisons, keeping signs and units explicit, calculating interval rates and avoiding unsupported conclusions about a surviving chronometer.

From seconds to angular error

Earth’s 360° rotation in 24 hours gives 15° per hour. This learning tool converts the magnitude of a time error into its idealised angular longitude equivalent, not a geographic position or a distance.

1 arcminutes of longitude

1 arcminute = 1/60 degree; four seconds of time = one arcminute. Direction, observational corrections and latitude-dependent ground distance are intentionally not calculated. Not for navigation.

Sources & references

  1. https://www.rmg.co.uk/stories/time/what-longitude

Where sources disagree, the disagreement is stated rather than resolved silently.

Exact collection examples

Arnold & Son No. 92 — dial and octagonal case, L2015-3486Marine chronometerJohn Arnold & Son marine chronometer No. 92c. 1795. The Clockmakers’ Museum record describes a spring-detent marine chronometer signed “John Arnold & Son, London No. 92 Invt. et Fect”. Its institutional identifier is L2015-3486; the historical museum number is 604.

Reviewed object views

Arnold & Son No. 92 — dial and octagonal case, L2015-3486
Arnold & Son No. 92 — dial and octagonal case, L2015-3486 — Clarissa Bruce / The Clockmakers’ Museum; © The Clockmakers’ Charity, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source
Arnold & Son No. 92 — exposed movement, L2015-3486
Arnold & Son No. 92 — exposed movement, L2015-3486 — Clarissa Bruce / The Clockmakers’ Museum; © The Clockmakers’ Charity, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

Matched to the official catalogue; no present operation or current display is asserted.

E. Barthet No. 10 — gimballed box, L2015-3496Marine chronometerE. Barthet marine chronometer No. 10c. 1840. Science Museum Group catalogues this chronometer as L2015-3496, historical Clockmakers’ Museum number 620. Its silvered dial has a central minute hand, subsidiary hours and seconds, and a thermometer marked for the Réaumur scale.

Reviewed object views

E. Barthet No. 10 — gimballed box, L2015-3496
E. Barthet No. 10 — gimballed box, L2015-3496 — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source
E. Barthet No. 10 — dial and canister
E. Barthet No. 10 — dial and canister — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source
E. Barthet No. 10 — exposed movement
E. Barthet No. 10 — exposed movement — © The Board of Trustees of the Science Museum / Science Museum Group / The Clockmakers’ Museum, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

Matched to the official catalogue; no present operation or current display is asserted.

Patek Philippe crystal-controlled chronometer, 1998-735Quartz marine chronometerPatek Philippe crystal-controlled marine chronometer1965–1984 (museum estimate). Science Museum Group identifies object 1998-735 as a quartz crystal-controlled marine chronometer made by Patek Philippe, Geneva, and supplied by Kelvin Hughes, London.

Reviewed object views

Patek Philippe crystal-controlled chronometer, 1998-735
Patek Philippe crystal-controlled chronometer, 1998-735 — © The Board of Trustees of the Science Museum / Science Museum Group, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source

Matched to the official catalogue; no present operation or current display is asserted.

Omega Megaquartz 2400 wristwatch, 1975-13Quartz wristwatchOmega Megaquartz 2400 Marine Chronometer wristwatch1950–1976 (museum range; not independently narrowed). Science Museum Group catalogues object 1975-13 as “Omega Megaquartz 2400 marine chronometer, 1950-1976”. The exact photograph shows a wristwatch with bracelet, not a boxed shipboard chronometer.

Reviewed object views

Omega Megaquartz 2400 wristwatch, 1975-13
Omega Megaquartz 2400 wristwatch, 1975-13 — © The Board of Trustees of the Science Museum / Science Museum Group, CC BY-NC-SA 4.0 as stated by source; commercial-use permission review pending. Official museum or publication source
OMEGA Megaquartz 2400 Marine Chronometer Steel & Gold Watch. Ca 1970’s ...
OMEGA Megaquartz 2400 Marine Chronometer Steel & Gold Watch. Ca 1970’s ... — marketplace.watchcharts.com, Rights unverified — web image search result, review pending. Web page (marketplace.watchcharts.com)

Matched to the official catalogue; no present operation or current display is asserted.

Further research

Verified reference libraryMuseum watch databasePlan a museum visitTechnical glossary

Photographs are hosted locally with source credits. Source-stated licences do not mean commercial permission is cleared. Preserve historical evidence: conservation before restoration before repair, with qualified professional assessment before operating fragile instruments.