The Astronomical Observatories of Jai Singh c9.s1.13

c9.s1.13

30. The Rām Yantra is the third of the stone instruments mentioned in the preface to the Zij Muḥammad Shāhī (page 13). The Paṇḍits say it was named after Rām Singh, a predecessor of Jai Singh’s. According to Hunter the instrument was also known as Ustuwani, which was the name given by al-Bīrūnī³ to an astrolabe on a cylindrical (orthographic) projection he devised. The Rām Yantra is a cylindrical instrument open at the top and having at its centre a pillar. The floor and the inside of the circular wall are graduated in scales of tangents for altitude and azimuth observations. The height of the wall from the graduated floor is equal to the distance from the circumference of the central pillar to the inside of the wall. To facilitate observation the floor is broken up into sectors (see figure 47 and plate XVII), and, consequently, as in the case of the Jai Prakāś, complementary buildings had to be constructed (see figure 41). The walls also are broken up, and one section of the wall corresponds to one sector. At Delhi there are 30 sectors, each of 6 degrees, in each building, but at Jaipur there are 12 sectors only, and their angle is 12 degrees in one instrument and 18 degrees in the other, the spaces between them being respectively 18 and 12 degrees.