'Yantralaya' commonly known as Jantar Mantar was a dream of Swai Jai Singh II, a great astronomer of India. Jantar Mantar was built between 1716. It is located within the city palace. Jantar Mantar contains a collection of instruments for observing the movements of sun, moon, planets and stars. It was built to develop the interest of the people in Astronomy. Maharaja Swai Jai singh was an able ruler and a brilliant astronomer. Commissioned by Emperor Muhammad Shah, to correct the existing astronomical tables and fix planetary positions anew, Sawai Jai Singh-II, accomplished the task in seven years and for this task, built the first stone observatory in Delhi in 1724 and in Jaipur in 1728. Not only did the masonry instruments suit his purposes, they also satisfied his architectural instincts. Apart from being a permanent memorial to his genius, is secured for him a place along with such distinguished observatory builders like Prince Ulugh Beg, Tycho Brahe and John Flam steed.