gnomon

As visitors hover over the work, functioning as a vertical gnomon, their shadows fall onto one of the panels, depending on the time of day.

The pole serves as a gnomon, the part of a sundial that casts a shadow to denote time.

A sundial uses a shadow cast by a thin rod called a gnomon on to a flat surface etched with different times.

A gnomon is the perpendicular part of a sundial, the part that casts a shadow.

Pytheas of Marseilles—who lived in the time of Alexander the Great, about 330 B.C.—measured the latitude of Marseilles by means of a gnomon, and found it to be about 42° 56′½.