What is a Marine Chronometer?
A marine chronometer is a watch that is accurate enough (what this means you will see later) to determine the longitude of a position on land and, more importantly, at sea. To get a general idea of what a marine chronometer is, you can look it up on Wikipedia:
Accuracy of a Marine Chronometer
It is difficult to find documents about the minimum requirements for a modern marine chronometer. The Observatory of the University of Besançon, which certified the Omega Marine Chronometers in the 1970s, allowed a daily deviation of 0.17 seconds in their requirements.
As a rule of thumb, a marine chronometer should not lose or gain more than 15 seconds per month (0.5 seconds per day, see Wikipedia or Britannica). It is also important that it is accurate under all sorts of different conditions, such as temperature or shocks. Using this rule, we can say that a mechanical marine chronometer is within +/- 3minutes per year.
A modern quartz marine chronometer is more accurate. About +- 5 to 25 seconds a year.
To give you a comparison, a normal quartz watch is within +/- 30 seconds/month and a good mechanical wristwatch is within 90 seconds/month.
Certificate for an Omega Marine Chronometer, issued by the Observatory of the University of Besançon. It is very interesting to see how the Omega Marine Chronometer, with a daily deviation of 0.004 s/d, is much more accurate than the required specifications of 0.170 s/d.
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