-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

-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.)