One of the five observatories of India built by Sawai Jai Singh, the leading astronomer of his times and the founder of Jaipur, Jantar Mantar of Jaipur is one of the largest and the best preserved. The collection of giant masonry astronomical instruments continues to provide accurate information even today and is comparable with the most sophisticated instruments of today. The Brihat Samrat Yantra Sundia is said to be the most imposing of all instruments and is 27 m high. It helps in the calculation of local time as its shadow moves up to 4 m in an hour. It also helps in ascertaining the various attributes of the heavenly bodies and is definitely the highlight and center of attraction of the observatory. 'Jantar-Mantar' in Sanskrit means, 'Magical Device'. It was built between 1728 and 1734 based on the blueprints drafted by Maharaja Jai Singh. Though Jaipur is based on Shilpa Shastra, the only deviation was that the holy area reserved by the Indian architectural treatise for the temple, was used by Jai Singh to make Jantar Mantar, his observatory, which showcases both the scientific and religious aspects of the genius king. The stone observation devices built here are of enormous dimensions and are unique for their precision, size and architectural perfection. Jai Singh founded Jai Praksh Yantra, which is set in the ground. A masterpiece in itself, it is one marble hemisphere about 4 m in diameter. It helps to ascertain the exact position of the sun on the coordinate lines, and hence the time. It lies in the centre of Jantar Mantar. There are 14 gnomes or shadow indicators in Jantar Mantar that are actually sundials and each have different functions attributed to them based on similar principles.