✹ About Friday 10 January 1661/62 Nate Lockwood on 11 Jan 2015 • Link I think it's difficult for us to comprehend difficulties of navigation before Harrison's clock. Around 1958 I sailed on three cargo ships as a cadet, something like a midshipman without any authority. Our only electronic navigation instrument was a RDF (Radio Direction Finder) which, to the best of my knowledge, was never used for navigation because, even after two or three days of cloudy weather our uncertainty of position was always less than the RDF's uncertainty. Thus we were dependent on dead reckoning. But computing our speed was much, much, better that that used by sailing ships (counting how many knots in a knotted line slip past one's fingers in some period of time measured with an hourglass) and our last known position was much more recent and accurate than it would have been before the chronometer. The route of many ships across the Atlantic would involve sailing south pretty much along the coasts until the altitude (angle with the horizon) of the North Star matched the altitude of the destination and then sailing East using dead reckoning to estimate Longitude and watching for land birds, floating debris such as tree branches and leaves, and the peculiar clouds that form over islands to forewarn the proximity of land. Still, groundings (and collisions) continue to occur to this day. In Sam's day many countries established their prime meridian based on the country's capital city and not all the charts had North at the top which further complicated exchanges of position.