You're likely referring to the gnomon on a shadow clock (also known as a sundial).

The gnomon is the part that casts the shadow, which is used to tell time. It's typically a rod or a triangular piece of metal or stone.

Here's a breakdown:

* The gnomon - The part that casts the shadow. Its length, angle, and orientation are crucial for accurate timekeeping.

* The dial - The surface on which the shadow falls. It is usually marked with hours and sometimes minutes.

The gnomon's shadow moves across the dial as the sun moves across the sky, and the position of the shadow indicates the time.