Abstract: Our concept of time is deeply rooted in ancient thought, which in Greco-Roman times already had two aspects: chronological time (χρονος) and auspicious time (καιρος). The essential instrument employed for both was the sundial, with its shadow markings by day, and its seasonal record of the constellations most closely associated with the solar passage at sunrise and sunset. The construction of sundials was undertaken with great care, and serve as models of Greek craftsmanship and
mathematical ingenuity. A particularly fine specimen, unearthed in 1865 from the buried ruins of Pompeii, and now housed in the Museo Archeologico Nazionale di Napoli, will form the focal point of discussion. In the lecture, the following topics will be considered: time-keeping and its significance in the Greco-Roman world, the devastation by Vesuvius of the ancient city of Pompeii, and the elements of mathematics employed in the analysis of the shadow markings, including a brief synopsis of the methods of recording the solar and stellar coordinates on the celestial sphere.