When I was more active collecting clocks in the 1990s, One of the members had a portable HP atomic clock. It was the size of a small suitcase. I think it was cesium based. What was interesting is that the actual timekeeping was done with a quartz crystal in a temperature controlled ‘oven’ The cesium fountain was used to keep things in sync ( think by measuring the radioactive decay.)
This clock was a bit obsolete at the time as precise timekeeping was being done with Hydrogen mazer. Cell towers also have highly accurate clocks, and precise locations, which are checked against the GPS satellites. I think it you can triangulate three towers you can find your exact location without seeing the birds. Getting the time from one of the birds needs relativistic corrections. Turns out that nothing is absolute and gravity does not exist as it is a dimensional illusion, like the other directions. It does not help that the past is infinitely slow, and the future infinitely fast. Finite infinities are so fun to play with. You can however find your land location precisely with a pendulum to measure the local gravitational constant, which will tell you the local time.
Ironically Nostradamus did his scrying with a pendulum on a tripod over a bowl of water acting as a mirror. I think Gravity is what separates the theories between Hawking and Kip Thorn. Personally I like the illusion of free will. It is weird to think that what I will write in the later part of this post already exist before I write it. Not that the paragraphs are written in order. ( tend to revise on the re-read before hitting send.)
The trick, with precise timekeeping is keeping the crystal at a common temperature. Why common quarts wristwatches are better timekeepers than the best mechanical chronometers. Even so If one was navigating with either one, it is best to keep a temperature log. Not to mention rate charts. The best watch/clock makers were metallurgists, who could measure the shape of the alloy over time and temprature and formulate the best solutions.
Ironically my mentor, said that in the 1940s and 1950s some of the best mechanical watches were made. That we know the 16th to 18th century alloys will be stable for about 500 years. That the post war alloys might be stable for a 1000 years or more. I forget how old the gears in the Prague clock (which I have seen in person.) they are 100s of years old, I think dating back to the 12th century when tower clocks were the tech like computers are now. Every city had one, and it went exponential in the 18th and 19th century when ever person had a watch or a clock. By the 20th century they were small and strong enough to strap to the wrist. (although queen Elizabeth the First had a bracelet with a watch in it.)
I always found it interesting that time is the easiest to measure but hardest to define. Temperature is easy to define but hard to measure. When I was writing the color test suite for Apple laser printers, I learned that color and temperature are the same thing. Now if someone could relate temperature to time …
