Which ancient device is considered the ancestor of the sundial?

The gnomon is recognized as the ancestor of the sundial because it fundamentally represents the essential principle behind how sundials work. A gnomon is a simple object, often a stick or a pole, that casts a shadow when sunlight hits it. The position and length of the shadow change throughout the day as the sun moves across the sky. This characteristic of the gnomon directly correlates with the basic functionality of a sundial, which is designed to use the sun's position and the shadow it casts to indicate the time of day.

While water clocks, hydromechanical clocks, and fire clocks use different mechanisms to measure time, they do not rely on the sun as a primary source of time measurement. Thus, choosing the gnomon highlights a direct lineage to sundials, emphasizing the ancient understanding of using natural elements to track time.