Let’s get one thing out of the way: a chronometer is different than a chronograph, though one can also be the other. We’ve heard the terms confused one too many times, and while we’ll forgive past sins, it’s time to know the difference once and for all. Read on.

The term chronometer comes from two Greek words, and roughly means “time measurer.” The word first came into use in the early 18th century with specific reference to timepieces designed for navigational use onboard ships. In those days — before LORAN, radar and GPS — getting a ship around the world, much less around a rocky peninsula, was a challenge to mariners. Two parameters are necessary to determine your exact whereabouts on the globe: latitude and longitude. Latitude, your position relative to the Poles, can be determined by the angle of the sun relative to where your boat is bobbing; that was discovered and used by sailors relatively early on.

To be marked as a chronometer shows a company’s commitment to accuracy.

The other parameter, longitude, is a bit trickier. So tricky, in fact, that a lack of accurate measurement caused ships to regularly miss their targets by hundreds, even thousands of miles — if they didn’t founder on rocks or break up on hidden reefs first. The British government took this problem very seriously, recognizing that a nation that could travel the seas with confidence would have a much easier time ruling the world. So they put up a large sum of reward money in a contest known as the Longitude Prize in 1714 to develop a means of reliably determining a ship’s precise location east to west.

While some contestants opted to use the stars as a guide, the use of an accurate timepiece, a chronometer, was more widely accepted as the real solution. By knowing the exact time at home port and by taking a “sun shot” at high noon using a sextant, a mariner could determine how many longitude lines of the globe he had crossed and thus determine his position. In 1761, 47 years after the British government offered their reward, the famed watchmaker John Harrison finally produced a reliable chronometer that would be accurate even through the temperature changes and rolling swells onboard a sailing vessel. He was awarded the prize (£14,315), the British Empire swelled, and the first true chronometer was born.

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Though the term chronometer was first used for marine navigational clocks, it was soon applied to any timepiece that met specific criteria for accuracy. The old adage, “a broken clock is right twice a day” helps illustrate the difference between precision and accuracy. Precision is the ability of a timepiece to be correct at any one moment. Accuracy is that timepiece’s ability to be precise day in and day out. This is no easy feat, given the malicious forces of gravity, temperature, magnetism and friction, which prey on the delicate mechanism inside a watch. It is also what makes earning the label “chronometer” so prestigious, even 250 years after Mr. Harrison’s timepiece debuted.

Chronometer Criteria

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Average daily rate:
-4/+6 seconds

Mean variation in rates:
2 seconds

Greatest variation in rates:
5 seconds

Rate variance for horizontal & vertical positions:
-6/+8 seconds

Thermal variation:
±0.6 seconds

Rate resumption:
±5 seconds

So what are the criteria for a true chronometer? The simplified answer is any mechanical timepiece that can keep time to within minus four to plus six seconds per 24-hour period. While that may not sound terribly accurate, consider that there are 86,400 seconds in one day. To be called a chronometer is to achieve a 99.9% accuracy rate. Name another mechanical device that can regularly lay claim to that tight a tolerance — especially one using technology originally developed centuries ago.

It’s actually not that difficult to achieve this level of accuracy in a timepiece that sits on a desk in a temperature-controlled room. But wristwatches, like marine chronometers, are exposed to a variety of conditions and positions. Therefore, to be a true chronometer is to be tested as such. By definition, a chronometer is only certified after it has been tested for fifteen days in five positions and at three different temperatures. The deviation rate of the movement is compared to a signal from an atomic clock and must maintain accuracy without differing too much between its position or temperature results.

The organization that bestows the chronometer certification is, as you might guess, Swiss, and they take their job very seriously. The Contrôle Officiel Suisse des Chronomètres, or COSC, is an independent agency established in 1973 whose sole focus is the testing and certification of chronometers. Watch companies send uncased movements to COSC for testing, where they are fitted with a generic seconds hand and have their self-winding components removed. The movements are then subjected to controlled experiments of position (crown up, crown down, lying flat and vertical) and temperatures. Only after the testing is completed and the results are verified can the movement be declared a chronometer.

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Only 3% of Swiss movements are chronometers, and most, predictably, are from a little brand named Rolex. Though not all Rolexes are chronometers, the sheer might of the brand and the number of watches they produce means that the vast majority of watches in the “minus 4 plus 6” club are theirs. One company that does sell only chronometers is the Swiss behemoth Breitling who, since 2000, has submitted all of its movements for certification (including its quartz pieces). At the other end of the size spectrum, the small British company Bremont also prides itself on only building chronometers.

Of course, this doesn’t mean a watch not certified as a chronometer is inaccurate. In many ways, the designation is largely a marketing ploy; there are plenty of brands, like IWC and Patek Philippe for example, producing very accurate watches that don’t bear the chronometer label. Still, to be marked as a chronometer shows a company’s commitment to accuracy. So even if you’re not solo circumnavigating the globe with your sextant, you can still be proud of your chronometer. And not confuse it with a chronograph.