How time served to measure the geographical position since hellenism

Abstract

The orthogonal grid of latitudes and longitudes on a spherical Earth dates back to Ptolemy’s book on Geography in the second century, and the suggestion to determine the longitude of a traveller dates back to the fourth century. Nevertheless, determining one’s own position while travelling remained a problem for two millennia. The solution was technically performed by the Harrison H4 clock in the eighteenth century, but the idea to measure longitudes by time had always been considered a computing exercise. In this paper, we discuss this classical exercise and the question whether and why it had not been applied. Primarily, we recall this example given by different authors and we will recognize differences in their values. Consequently, we will ask about their sources to trace the transfer and transformation of knowledge, since we will finally have to conclude that their descriptions of the measurement were not based on observations but rather that the numerical values were constructed. We further examine the accuracy of the method, investigating if ancient data were sufficient to apply it at all. Summarizing, we can only wonder why scientists for so many centuries praised the method’s elegance without collecting the data and applying the method.