In 1077, he was dispatched on a diplomatic mission to the Liao Dynasty of the Khitan people to the north, sharing ideas about calendrical science, as the Liao state had created its own calendar in 994 AD. In a finding that reportedly embarrassed the court, Su Song acknowledged to the emperor that the calendar of the Khitan people was in fact a bit more accurate than their own, resulting in the fining and punishment of officials in the Bureau of Astronomy and Calendar. Su was supposed to travel north to Liao and arrive promptly for a birthday celebration and feast on a day which coincided with the winter solstice of the Song calendar, but was actually a day behind the Liao calendar. Historian Liu Heping states that Emperor Zhezong of Song sponsored Su Song's clocktower in 1086 in order to compete with the Liao for "scientific and national superiority." In 1081, the court instructed Su Song to compile into a book the diplomatic history of Song-Liao relations, an elaborate task that, once complete, filled 200 volumes. With his extensive knowledge of cartography, Su Song was able to settle a heated border dispute between the Song and Liao dynasties.