Named for Zu Chong-Zhi (492–500), a distinguished astronomer-mathematician during the period of the Liu-Sung dynasty. He improved the calendrical system of his time. He constructed a bronze compass that can rotate freely while pointing in the same direction. He also ascertained that the ratio of the circumference to the diameter of a circle is between 3.1415926 and 3.1415927. (M 4419) _ _.