How do marine chronometers determine longitude?

A marine chronometer is a precision portable timepiece carried on ships to determine longitude through celestial navigation. By comparing Greenwich Mean Time with the local time found by observing celestial bodies, navigators can calculate their east-west position on Earth. This breakthrough solved a critical historical navigation problem that previously caused major naval disasters.

What we know

• A marine chronometer is used to determine longitude by comparing Greenwich Mean Time and the time at the current location found from observations of celestial bodies. [2]
• English clockmaker John Harrison invented the marine chronometer after decades of persistent experimentation and testing. [2, 4, 5]
• The British Parliament offered financial rewards of up to 20000 pounds under the 1714 Longitude Act following the Scilly naval disaster of 1707. [4]
• To calculate a position line using a chronometer, a navigator measures the angle between the sea horizon and a celestial body with a sextant and notes the exact time. [5]

Still open

• John Harrison never received the full financial reward offered by Parliament due to political rivalries.

Where to go next

• whoWho was John Harrison?
• what caused itWhat was the Scilly naval disaster of 1707?
• how it workedHow does celestial navigation work?
• what followedHow did satellite navigation replace chronometers?
• the bigger pictureWhat is the history of longitude?
• an unexpected connectionHow did a book on theology describe marine timepieces early on?An early theoretical description of a marine chronometer appeared in a 1713 theology book titled Physico-Theology written by English cleric William Derham.

Sources

• [1]marine chronometer (Wikidata Q2307829) · CC0 1.0
• [2]Marine chronometer · CC BY-SA 4.0
• [3]History of longitude · CC BY-SA 4.0
• [4]John Harrison · CC BY-SA 4.0
• [5]Longitude by chronometer · CC BY-SA 4.0