At the tip of the Great Samrat Yantra is the Shasthansa Yantra.. It consists of a 60 degree arc built in the plane of meridian within a dark chamber. The instrument is used for measuring the declination, zenith distance, and the diameter of the sun. As the sun drifts across the meridian at noon, its pinhole image falling on the Sasthansa scale below enables the observer to measure the zenith distance, declination, and the diameter of the sun. The image formed by the pinhole on the scale below is usually quite sharp, such that at times even sunspots may be seen on it. It has its origin in the Islamic tradition of astronomy, invented by Abu Mahmud al-Khujandi in the late 10th century. The exterior of the Shasthansa Yantra is seen in the right hand portion of the photo above, while the diagram to the right shows how it functions.