Portable vertical sundial, ca. 250 A.D. Oxford, Museum of the History of Science, inv. 51358 Portable sundial calibrated for a variety of latitudes. The curved piece rotating above the smaller disc is a combination between an hour-scale and a gnomon, to indicate the date. Suspended in the vertical plane, it marked the hour. The back of the larger disc carries the latitudes of 30 provinces in the Roman Empire.