Observatories have stood on this spot since the Yuan dynasty. Located in a former watchtower at Jianguomen, this one houses a reproduction of a Ming dynasty star map, and stargazing instruments like armillary spheres and a quadrant altazimuth. These instruments were designed in the 17th and 18th centuries by Jesuit missionaries who sought to use their knowledge of astronomy to win the court's trust and admiration, with the ultimate goal of converting the emperor and his subjects to Catholicism. The Jesuits also sought to advance their cause by teaching the Ming how to make cannons. The juxtaposition of baroque astronomical instruments on the observatory's deck and the sea of modern high-rises beyond creates a quintessentially Beijing vista - one that is most picturesque after a snowstorm. Try to imagine what the night sky would have looked like from the observatory before pollution blocked it out. Special programs are arranged here to mark celestial events like the appearance of comets.