A commenter to the wikipedia page I posted about the vancouver 9 o'clock gun makes a compelling point: it was all about providing a time signal for ships in port to set their onboard clocks.

That makes sense. Ships used time to help get their location back then and presumably even a second deviation could put you off course.

Ships also used Greenwich Mean Time (which this cannon is tied to) and astrological data for local time (which this cannon also used for its time).

But why down to the second? I can see why syncing clocks +/- 30 seconds could be useful (train schedules, etc.) but this chart is about being within a second.

-Surely if you are going through the trouble of setting your clock, you might as well do it accurately?

Also, as suggested by others (I should have thought of that!) - ships used chronometers to establish their longitude.

(At 56 degrees north, which Edinburgh lies on, a timing error of a second translates into 18 meters or so - an order or two of magnitude below the accuracy of even the best of celestial navigators - but still, might as well make the starting conditions as good as possible.)

NTP nerds before NTP