What is the purpose of a gnomon found in astronomical instruments?

A gnomon is one of the oldest tools used in astronomy to measure time and determine the Sun’s position. The word comes from Greek, meaning “indicator” or “one who knows.” Its purpose is simple but powerful: a gnomon casts a shadow when sunlight falls on it, and by studying the length and direction of that shadow, astronomers can find the time of day, the solar altitude, and even the solstices and equinoxes. 

In ancient observatories and sundials, a gnomon is typically a vertical rod or a triangular blade fixed perpendicular to a horizontal surface. As the Sun moves across the sky, the shadow moves too. 

Measuring this change helps calculate the apparent motion of the Sun and the latitude of a location. Famous historical instruments such as the Jantar Mantar observatories in India used large gnomons for accurate celestial measurements long before modern clocks existed. 

So, the main purpose of a gnomon in astronomical instruments is to serve as a shadow-casting pointer that converts sunlight into measurable information about time and the Earth’s orientation.