Marine chronometer

A highly accurate timekeeper carried aboard a ship to determine longitude by comparing its retained reference time with local solar time.

Greenwich Mean Time: Mean solar time measured from the Greenwich meridian, historically used as a global reference. A ship compared its chronometer’s Greenwich time with local solar time.
Longitude Act 1714: A British law offering rewards for practical methods of determining longitude at sea. Its prize created the incentive that drove the marine chronometer’s development.
Longitude: Angular distance east or west of a reference meridian, measured in degrees. Understanding the quantity to be found clarifies why accurate time matters.
HMS Beagle: A British survey ship best known for carrying Charles Darwin on its 1831–1836 voyage. Its voyage illustrates chronometers in long-distance surveying and scientific travel.
Royal Observatory, Greenwich: A British astronomical observatory founded in 1675 at Greenwich, London. Its meridian and time became central reference points for navigators using chronometers.
Equation of time: The difference between apparent solar time and mean solar time caused by Earth’s orbit and axial tilt. It explains why sundial time must be corrected when compared with mean time.
John Harrison: An English clockmaker who developed a series of marine timekeepers, culminating in the H4 watch. His instruments demonstrated that portable clocks could retain useful time at sea.
Mean solar time: Time measured by a fictitious Sun moving uniformly along the celestial equator. Chronometers kept mean time, avoiding the irregular daily motion of the apparent Sun.
Hydrographic survey: The measurement and description of bodies of water and their shorelines for navigation and other uses. Surveyors used chronometer-based longitude to position soundings and coastal features.
Prime meridian: The reference meridian assigned zero degrees longitude for a coordinate system. Chronometer navigation depended on a known reference meridian, later internationally fixed at Greenwich.

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