Here's why:
* What is a gnomon? A gnomon is a vertical object used to tell time by the length and direction of its shadow. It can be a simple stick, obelisk, or any other upright structure.
* How it works: The sun's position in the sky changes throughout the day. As the sun moves, the shadow cast by the gnomon changes in length and direction.
* The surface: The shadow is cast on the surface beneath the gnomon, often a flat area of ground or a specially marked dial.
So, the shadow of a gnomon doesn't just "cast" in the air - it falls on a specific surface where it can be observed and measured.