IAA/Israel Museum

A dial with style. This limestone dial, excavated from Jerusalem’s Jewish Quarter, displays one of the most prevalent styles found among first- and second-century C.E. sundials—a curve divided into sections. Called conical dials, their simple shape belies an advanced understanding of astronomy, geography and mathematics. To measure accurately the hours in a day, sundials must allow for the variations in the sun’s apparent path across the sky over a year at a particular site. The hour lines have to be far enough apart to allow for seasonal extremes, since the sun crosses the sky at a lower angle to the horizon during winter, reaching its nadir during the winter solstice, and at a higher angle in summer, reaching its zenith at the summer solstice. The placement of the lines must also reflect the latitude of the dial’s intended location, since the sun will appear higher in the sky the closer one is to the equator and lower the nearer one is to the earth’s poles. The length of each dial’s gnomon, the object used to cast the shadow on its face, also has to be carefully calculated, along with its angle to the dial’s face. For this dial, which lies on its back in this photo, the gnomon was probably a bar set horizontally in its top, the part facing left.