> “Back in the day” we used WWV and the kitchen clock for that sort of thing……


Yes, not much has changed. I use multiple methods to measure 60 Hz in order to 
gain confidence in the results. Besides the picPET, I've used a commercial 
TrueTime TFDM (Time/Frequency Deviation Meter) and also a plain old kitchen 
clock (synchronous motor, wall clock).

Example: I took photos of the kitchen clock precisely 30 seconds after each 
quarter hour. Here's the short animated GIF of that run; you can see how the 
wall clock wanders from 0 to 5 seconds ahead of the UTC reference clock (seen 
in the background):


For alert readers: the +/- 1 second jitter in the reference clock is due to 
drift and latency in the PC scripts used to trigger the photo capture. Also 
sunrise (Pacific time) can be seen in the background starting about 1300 UTC.


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