What Does Chronometer Certified Mean?

Have you ever wondered what is typed on watch Dials saying “Chronometer“, “Chronometer Certified” or normally on a Rolex Dial ” Superlative Chronometer Officially Certified“.

chronometer Sub
Rolex Submariner Dial – image (HODINKEE)

The word Chronometer is derived from the Greek words, Chronos ( meaning Time ) & Meter ( meaning Counter), Jeremy Thacker came up with the name referring to his invention of a clock enclosed in a vacuum chamber in England 1714.

There were several navigation disasters at that time & most famously the Scilly naval disaster of 1707 which led to the loss of four warships of the British Royal Navy fleet & Up to 1550 sailors lost their lives, making the incident one of the worst maritime disasters in British naval history.

Navy Disaster
Illustration of The Scilly Naval Disaster of 1707 

These kind of accidents was due to the lack of accurately calculating their positions at sea and inadequate compasses.

In order to solve these issues, The Longitude Act was created in the United Kingdom in 1714 offering money rewards to someone who would develop a simple & practical way in order to calculate the ship positions (Longitude) at sea to simplify navigation.

John Harrison invented the Marine Chronometer & was able to solve the problem of Marine Navigations.

John Harrison, English inventor and horologist, 1767.
John Harrison

The Marine Chronometer is a timepiece that is very precise & accurate enough to be used as a portable time standard. which helped navigators determine their positions at sea by means of celestial navigation.

John Harrison Marine Chronometer

The Term is used today to describe accurate wrist watches that are tested & certified based on several precision standards.

omega MC
OMEGA Seamaster Master Chronometer

The most famous certifying institute is the Contrôle Officiel Suisse des Chronomètres (COSC) which is responsible for certifying the accuracy and precision of wristwatches in Switzerland.


The COSC tests wristwatches ( Quartz & Mechanical ), Pocket Watches & Fixed time devices like clocks, only manufactured in Switzerland.

The examination lasts between 15 – 19 days depending on the watch type & is only then certified if they meet all the 7 criteria which are:

  • average daily rate
  • mean variation in rates
  • greatest variation in rates
  • the difference between rates in horizontal and vertical positions
  • largest variation in rates
  • variation in rate depending on temperature
  • rate resumption
COSC Chronometer Certificate

That doesn’t mean only Swiss made watches are the only accurate watches out there. In Germany for example they have set their own testing facility at the Glashütte Observatory.

Glashütte Observatory
Glashütte Observatory

In the end when buying a watch, it doesn’t necessarily have to be a Chronometer Certified, as several watch brands have developed serious testing in-house facilities & can easily surpass the testing of COSC. However Brands like Rolex & Tudor are always committed to have their watches certified.

tudor chron 2
Tudor Watch 


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