The Chronometer

The chronometer is a timepiece with a nearly constant rate. It is used for making accurate estimations of the correct time in the absence of time reference signals as available from radio broadcast services or satellite systems. Time on board of a ship is important for example to: determine speed with a traditional log, plan observations for celestial navigation, determine longitude by methods of celestial navigation, plan recordings or captures of radio transmissions (e.g. weather forecasts) The determination of position by Celestial Navigation methods requires correct (absolute) time information down to the second. Roughly an error of 4 seconds corresponds to a position error of 1 nautical mile.

Absolute time was introduced into marine navigation not before the 17th century, when John Harrison (1693-1776) made the first accurate and portable clocks, which enabled determination of position by methods of Celestial Navigation. The chronometer on board, would kept track of the time in England, and by comparing this with local time, a navigator could find the longitude of his ship. Solving this "longitude problem" produced a revolution in European exploration and mapping. Before, it was impossible to determine the exact longitude of an unknown location and "time" on board of a ship was only used to determine speed and travelled distance used for dead reckoning navigation. Time intervals were measured with a sandglass. Also board time was "measured" by timing fixed time intervals with a sandglass.

Today, at least one accurate chronometer will be on board of each offshore sailing vessel. The chronometer is normally maintained to UTC time and used as reference clock for other on-board clocks. The board clock will show local time, but it is synchronized with the UTC chronometer at regular intervals.

For taking altitudes of celestial bodies, a separate stop-watch is used, started at a known chronometer time. The elapsed time of each sight is added to this chronometer starting time to obtain the UTC time of the sight.

Chronometer Error and Chronometer Correction

The chronometer error is the amount of time by which the chronometer time differs from the correct time to which it was set, usually UTC (Universal Time Coordinated). The chronometer error is expressed to an accuracy of one second and has an algebraic sign according to:

  chronometer-error = chronometer-time - correct-time

The chronometer correction is the amount that must be added algebraically to the chronometer time to obtain the correct time. It is numerically equal to the chronometer error, but of opposite sign:

  chronometer-time + chronometer-correction = correct-time

The chronometer is checked at regular intervals with an accurate time signal as obtainable from satellite or radio signals. These checks should be recorded in a table similar to the one shown below and stored with the chronometer. With this record it is possible to make accurate estimations for times after the last chronometer check.

From the chronometer rate and the last recorded chronometer correction, the correct time for a current instance may be calculated:

  correct-time = chronometer-time + chronometer-correction + chronometer-rate * number-of-days

The spring-driven ship clocks constructed by John Harrison in the 17th century had a rate of about 0.750s/day. Modern electrical quartz-crystal-based chronometers may have a rate better than 0.010s/day. In this case the rate should nevertheless be recorded with the appropriate precision, and checked for unexpected changes, which may indicate a deterioration of battery capacity or some other need for service.