How to Make a Modern-Day Chronometer?

The definition of the chronometer in our mind is an extremely accurate timepiece, correct? But, what’s in detail? You know that the chronometer 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, knowing what time it was in Greenwich, England (hence, the use of GMT) at the same time. Use simple arithmetic to find the time difference and you could find out how far west or east you were. That’s longitude.
But knowing the time in Greenwich was the rub. You may want fake watches on board your ship that was set to Greenwich time. OK, that’s simple enough. You set the particular clock to Greenwich time before you leave England. But how do you know that clock isn’t running slow or fast? Well, you have to trust the accuracy of said clock. And that’s where chronometers come in.
Normally speaking, the movement that’s been certified by COSC and achieved chronometer status is accurate to -4/+6 seconds per day. But there’s a little more to it than that.
The testing it takes to achieve a COSC certificate 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:
And to achieve this kind of performance, there are many other physical effects and circumstances that need to be mitigated, avoided, or overcome, with regard to individual components and their assembly.
The dimensions of each component have to be controlled. Design tolerances – the degree to which each piece part is dimensionally identical to its siblings in a manufacturing lot, and to the original design dimensions – must be maintained. This also relates to how each component mates to the parts it interacts with. Temperature effects include viscosity changes in lubricants and thermal growth or shrinkage of component parts.
There are still going to be infinitesimal differences in the build of each movement, even though it is properly accounted for. These are due both to infinitesimal differences in the individual components used to build each movement, and minute differences in alignments – even with locating pins and shoulders.
These small differences are enough to cause differences in how each movement runs before adjusting. Therefore, a watchmaker is needed to make those final adjustments and bring the movement into spec so it can pass the COSC tests. However, you can see how making and adjusting a replica watch movement with over 200 parts to perform within seconds per day is no small feat.