Greek and Roman portable sundials. An ancient essay in approximation
The author reports about nine portable sundials that have survived from Antiquity. These devices show the usual subdivision of the time from sunrise to sunset into twelve equal hours. (In contrast to the modern 24 equal hours from midnight to midnight.) Fixed sundials work exactly in principle, but the portable ones show the time only approximately. The author explains their working in detail and calculates their error according to the time of the day, the season of the year, and the observers latitude. Exact values are obtained at the equinoxes, but otherwise the deviation from the exact time may be as much as 15 minutes (in the locations of the antique world). This can be neglected if only the approximate hour of the day is of interest.
scientific article; zbMATH DE number 1556787