During a diplomatic mission to the
Ming in
Beijing in early 1631, Jeong stopped at
Dengzhou (now Penglai) on the northern coast of
Shandong. The normal route was overland, but the Koreans were obliged to travel directly across the
Yellow Sea owing to the northern war zones created by the initial phases of the
Manchu conquest of China. There, he met with the province's Christian governor under the Ming
Ignatius Sun, who introduced him to the
Jesuit interpreter
João Rodrigues. Rodrigues was working with
Gonçalo Teixeira-Correa to train Sun's forces in the use of
European-style cannon. He made a personal gift of his
telescope to Jeong, which became the first such device known in Korea. He also laded Jeong down with Jesuit texts on
astronomy and
other sciences including
Alenio's
Record of Foreign Lands (,
Chikpang Oegi); a treatise on artillery and its use; and a guide to European customs and manners, as well as works on
Christianity. Despite also receiving some European firearm, Jeong praised the telescope most highly, as he understood
its importance for warfare. He also had his assistants
Yi Yeonghu (, ) and Colonel
Jeong Hyogil (, ) speak with Rodrigues in greater detail, Yi about geography and Col. Jeong about Western firearms and cannon. A record survives of Yi's conversation. He was most curious about whether or not China—whose native name
Zhōngguó () literally means "The Central Realm"—did in fact occupy the middle of the Earth. Rodrigues replied that, since the Earth was a sphere, every country could truthfully claim their land as its center. ==Legacy==