I spent 8 or 10 or 15 hours listening to the audiobook Longitude, and most of that was how much trying to make a clock that worked at sea consumed John Harrison's life. All of this made me itch to take advantage of centuries of development and buy a practically free, practically perfect pocket watch. It takes two clocks to work out longitude. Here is what you do: Keep one set to the time at the Prime Meridian, where it is 0* and they use Greenwich Mean Time. Then, at local noon, set the other clock. After you subtract GMT from local time and divide by twenty-four, multiply your result by 360* and you are all done! The only thing is, Greenwich uses Mean Time, averaging all the noons in a year, and we are using solar time directly. There's an Equation of Time for that, but I haven't learned it. I made a little reference card for myself and put it into the locket watch.
*You can multiply by 15 but then remembering that the sun takes 24 hours to visit 360* of Earth wouldn't help you remember how to calculate longitude.