A gnomon is a device used to indicate time, typically found in sundials. It consists of a vertical stick or a triangular blade that casts a shadow on a marked surface. As the sun moves across the sky, the position of the shadow changes, allowing people to tell the time based on the angle of the shadow. In addition to its use in sundials, the term "gnomon" also refers to a mathematical concept. In geometry, a gnomon is a shape that, when added to a particular figure, creates a larger similar figure. This concept is often associated with the work of the ancient Greek mathematician Euclid.