Zu Geng or Zu Gengzhi (; ca. 480 â ca. 525) was a Chinese mathematician, politician, and writer. His courtesy name was Jingshuo (). He was the son of the famous mathematician Zu Chongzhi. He is known principally for deriving and proving the formula for the volume of a sphere. He additionally measured the angular distance between Polaris and the celestial north pole, which was greater 1600 years ago than it is today.