The Technology of Greco-Roman Time Reckoning

By Alexander Jones

"Living by the clock" was essentially unknown in Classical Greece. Under the Roman Empire, however, precise time specifications were common, witnessed by dinner invitationstimes of births in horoscopesand other astrological consultationsBureaucracy was a leader in precision; in the third century BCE the Ptolemaic mail service recorded hours of arrival and departure of couriers.

On September 30, Alexander Jones gave a fireside chat for Friends of ISAW about some devices invented by Greek scientists and craftsmen to tell time. The Greeks divided the daytime into twelve hours, and the nighttime into twelve hours, so day hours were longer in the summer and shorter in the winter, and night hours the reverse. They also required all the hours within a day to be equal in length, understanding this in terms of a cosmology that treated the heavens as an immense spherical shell revolving around the Earth, carrying the Sun and stars on its spinning surface. An hour of day was defined as one-twelfth of the part of the Sun's daily path above the horizon.

Greco-Roman sundials projected the Sun's daily motion as a shadow on a stone surface engraved with a grid of lines: "day curves" representing the Sun's track from sunrise to sunset at different seasons of the year, crossed by "hour curves." In one common type, the surface was a spherical shell with the tip of the shadow caster at its center, so the sundial became an image of the celestial sphereThere existed many varieties, exhibiting the geometers' ingenuity. One of the most spectacular public displays of sundial geometry was the Tower of the Winds in Athens (c. 100 BCE), with sundials on its eight walls. Sundials were common throughout the Greco-Roman world; more than five hundred have survived. 

Another technology was the water clock. To make a water clock that measured hours in the same way as a sundial, the inventor had to discover how to keep the flow of water uniform, and how to translate this flow into seasonally varying hours. The "anaphoric clockcombined a regulated apparatus for water flow with a revolving dial representing the celestial sphere with the sun and stars. A sophisticated grid of wires in front of the dial gave the read-off of the time. Our main evidence for these clocks is Vitruvius; they were too delicate to survive. Fortunately we have parts of dials from anaphoric clocks, one of which is engraved with the constellations.