A gnomon is the part of a sundial that casts a shadow, helping to indicate the time of day. It is typically a vertical stick or a triangular blade positioned at an angle. As the sun moves across the sky, the shadow it casts changes position, allowing users to read the time based on the shadow's length and direction. Gnomons have been used since ancient times, with early examples found in Egypt and Greece. They are also used in modern applications, such as in solar energy systems and architecture, where they help in understanding sunlight patterns and optimizing light exposure.