In a Roman mosaic from antiquity, a man on a street studies the sundial atop a tall column. The sun alerts him to hurry if he does not want to be late for a dinner invitation.

Sundials were ubiquitous in Mediterranean cultures more than 2,000 years ago. They were the clocks of their day, early tools essential to reckoning the passage of time and its relationship to the larger universe.

The mosaic image is an arresting way station in a new exhibition, ”Time and Cosmos in Greco-Roman Antiquity,” that opened last week in Manhattan at the Institute for the Study of the Ancient World, an affiliate of New York University. It will continue until April.