{"id":10684,"date":"2018-06-23T06:00:20","date_gmt":"2018-06-23T13:00:20","guid":{"rendered":"https:\/\/www.bobswatches.com\/rolex-blog\/?p=10684"},"modified":"2019-02-23T09:33:12","modified_gmt":"2019-02-23T17:33:12","slug":"goes-making-modern-day-chronometer","status":"publish","type":"post","link":"https:\/\/www.bobswatches.com\/rolex-blog\/resources\/goes-making-modern-day-chronometer.html","title":{"rendered":"What Goes in to Making A Modern-Day Chronometer?"},"content":{"rendered":"

The quick and dirty definition of a chronometer is an extremely accurate time piece, right? But how accurate is accurate? Well, the chronometer as we know it today grew out of the pursuit of a solution to the problem of determining longitude at sea. That solution, it turned out, was to know when it was noon at your location on the open ocean and, simultaneously knowing what time it was in Greenwich, England (hence, the use of GMT<\/a>). Use simple arithmetic to find the time difference and you could figure out how far west or east you were. That\u2019s longitude. (North and south was figured by sighting the North Star at night \u2013 a task quite separate from time measurement. )<\/p>\n

\"The
The GMT Master II Ref. 116718, the G meaning Greenwich<\/figcaption><\/figure>\n

But knowing the time in Greenwich was the rub. You needed a clock on board your ship that was set to Greenwich time. OK, that\u2019s easy enough. You set the special clock to Greenwich time before you leave England. But how do you know that clock isn\u2019t running slow or fast? Well, you have to trust to the accuracy of said clock. And that\u2019s where chronometers come in.<\/p>\n

We won\u2019t go into the technical details of what John Harrison did to design a clock good enough<\/a> to ultimately won the Longitude Prize, but win it he did.<\/p>\n

And now Contr\u00f4le Officiel Suisse des Chronom\u00e8tres, or COSC, the Official Swiss Chronometer Testing Institute, is the organization which handles certifying the accuracy and precision of wristwatches in Switzerland \u2013 and by extension, the world.<\/p>\n

\"Rolex
Rolex Caliber 3135 Parts<\/figcaption><\/figure>\n

Basically, a movement that\u2019s been certified by COSC and achieved chronometer status is accurate to -4\/+6 seconds per day. But there\u2019s a little more to it than that.<\/p>\n

The testing it takes to achieve a COSC certificate<\/a> is based on ISO standard 3159. A movement is tested over fifteen days, in five positions, at three different temperatures. Measurements are made each day, and are compared to two atomic clocks. The standards for achieving a chronometer certificate are as follows:<\/p>\n