ssical era of Indian astronomy begins in the late Gupta era, in the fifth–sixth centuries. The Panchasiddhantika (Varahamihira, 505 CE) approximates the method for determination of the meridian direction from any three positions of the shadow using Gnomon or Sanku. Once, while visiting the court of Emperor Muhammad Shah, Maharaja Jai Singh II of Jaipur OVERHEARD a loud argument about how to calculate the most astronomically advantageous date for the purpose of the emperor beginning a journey. To the Maharaja, the debate highlighted the need for education in the FIELD of astronomy and for an OBSERVATORY that could make accurate astronomical calculations. The idea for the Jantar Mantars or calculation instruments was born. The Jantar Mantar CONSISTS of a number of structures in stone, brick and marble, each of them marked with astronomical scales and designed to serve a specific purpose. Of the observatories originally built at Delhi, Jaipur, Mathura, Ujjain and Varanasi, all observatories still exist except the one in Mathura. Among the devices used for astronomy was Gnomon, known as Sanku, in which the shadow of a vertical rod is applied on a horizontal PLANE in order to ascertain the cardinal directions, the latitude of the point of observation and the time of observation. This device finds mention in the works of Varahamihira, Aryabhatta, Bhaskara, Brahmagupta, among others. The armillary sphere was used for observation in India since early times, and finds mention in the works of Aryabhatta (476 CE). The Goladipika—a detailed treatise dealing with globes and the armillary sphere was composed between 1380– 1460 CE by Parameswara. Probably, the celestial coordinates of the junction stars of the lunar mansions were determined by the armillary sphere since the seventh century. There was also a celestial globe rotated by flowing water.