technically it's "electromechanical" but does not use GPS, WiFi, quartz, or atomic methods. To make thigns this accurate you need extremely good temperature compensation, close-to-frictionless bearings, run in a vacuum, use a pair of pendulums, etc.
When you reach that level of accuracy, you end up basically building a sensitive measurement device that is influenced by second and third order terms like subtle changes in the shape of the earth.
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https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clo...
technically it's "electromechanical" but does not use GPS, WiFi, quartz, or atomic methods. To make thigns this accurate you need extremely good temperature compensation, close-to-frictionless bearings, run in a vacuum, use a pair of pendulums, etc.
When you reach that level of accuracy, you end up basically building a sensitive measurement device that is influenced by second and third order terms like subtle changes in the shape of the earth.