twelve o'clock
What we do is this : We set our chronometers at Greenwich , and thus keep the Greenwich true time wherever we go . The greatest attention is paid to the chronometers , to see that they receive no injuries ; and usually there are two , and often more of them , to compare one with another , in order to see that they go well . When in the middle of the ocean , for instance , we find the true time of the day at that spot , by ascertaining the height of the sun . This we do by means of our quadrants , or sextants ; for , as the sun is always in the zenith at twelve o'clock , nothing is easier than to do this , when the sun can be seen , and an arc of the heavens measured . At the instant the height of the sun is ascertained by one observer , he calls to another , who notes the time on the chronometer . The difference in these two times , or that of the chronometer and that of the sun , gives the distance in degrees and minutes , between the longitude of Greenwich and that of the place on the ocean where the observer is ; and that gives him his longitude . If the difference is three hours and twenty minutes , in time , the distance from Greenwich is fifty degrees of longitude , because the sun rises three hours and twenty minutes sooner in London , than in the fiftieth degree of west longitude . ''