Xu Guangqi or Hsü Kuang-ch'i, also known by his baptismal name Paul or Paul Siu, was a Chinese agronomist, astronomer, mathematician, politician, and writer during the late Ming dynasty. Xu was appointed by the Chinese Emperor in 1629 to be the leader of the Shixian calendar reform, which he embarked on with the assistance of Jesuits. Xu was a colleague and collaborator of the Italian Jesuits Matteo Ricci and Sabatino de Ursis and assisted their translation of several classic Western texts into Chinese, including part of Euclid's Elements. He was also the author of the Nong Zheng Quan Shu, a treatise on agriculture.