@JonB remember, they don't have to be perfect -- they just have to be better than anything else. According to one of the articles I read, the pre-cesium definition of a second was based on a solar day. When they invented the cesium clock, they found that it's measurement was so close to the previous definition that the error was below any consumable measurement.
For some reason, this discussion reminds me of a story my mom told me about when she was in high school. One of the teachers that no one liked wore a shiny round brooch. My mom and her friends would catch the sunlight on their wristwatches, and deflect it toward the brooch so that it would hit the teacher in the eye.
I guess this was the sort of thing that teenage girls in parochial schools did for fun in the 1950s....
