Old instruments
Longitude by Chronometer
Direct answer
Longitude by Chronometer is a longitude determination from the difference between local celestial time and preserved reference-meridian time.
Longitude by Chronometer is a time-difference longitude method used in historical astronomy, timekeeping, or navigation.
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At a glance
- Found in
- Historical Almanacs & Instruments
- Related
- Dead Reckoning, Latitude by Meridian Altitude, Noon Sight, Star Sight
Longitude by Chronometer is a time-difference longitude method used in historical astronomy, timekeeping, or navigation.
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The story behind Longitude by Chronometer
Reliable marine chronometers made time-based longitude routine in the nineteenth century.
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Why Longitude by Chronometer matters
Longitude by Chronometer matters because Earth rotates fifteen degrees per hour, making accurate time difference an angular east-west measure.
Longitude by Chronometer in practice: an example
A four-hour local-reference time difference corresponds to sixty degrees of longitude before east-west assignment.
Where readings of Longitude by Chronometer go wrong
Clock error of four seconds represents roughly one arcminute of longitude at the equator, so rate knowledge matters.
Sources and editorial method
Research references used for this entry include:
- National Geospatial-Intelligence Agency: The American Practical Navigator - Official modern reference for navigation principles, celestial observations, sight reduction, and navigational tables.
- Royal Museums Greenwich: A Unique History of Timekeeping - Museum overview of marine chronometers, astronomical regulators, and the development of precision timekeeping.
- Source
- The Great Almanac Dictionary
- Category
- Historical Almanacs & Instruments
- Last updated
- 2026-09-26
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Common questions
What is Longitude by Chronometer?
Longitude by Chronometer is a longitude determination from the difference between local celestial time and preserved reference-meridian time.
What do people get wrong about Longitude by Chronometer?
Clock error of four seconds represents roughly one arcminute of longitude at the equator, so rate knowledge matters.
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