Zu Chongzhi

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Zu Chongzhi (祖冲之, pinyin Zǔ Chōngzhī, Wade-Giles Tsu Ch'ung-chih) (429–500) was a Chinese mathematician and astronomer during the Liu Song and Southern Qi Dynasties (of the Southern Dynasties).

His ancestors hailed from Qiu district, Fanyang Commandery (part of modern Beijing). To flee from the ravages of war, Zu Chongzhi'grandfather Zu Chang moved from Hebei, in north China, to south of the Yangtze River, as part of the massive population movement during the Eastern Jin Dynasty. Zu Chang at one point held the position of "Minister of Great Works" (Dàjiàngqīn) within the Song Dynasty (420-479) and was in charge of government construction projects. Zu Chongzhi's father also served the court and was greatly respected for his erudition.

Zu Chongzhi was born in Jiankang (today Nanjing). His family had historically been involved in astronomy research, and from childhood Zu Chongzhi was exposed to both astronomy and mathematics. When he was only a youth his talent earned him much repute. When Emperor Xiaowu heard of him, he was sent to an Academy, the "Huálín Xuéshěng", and later at the Imperial Nanking University (Zongmingguan) to perform research. In 461 in Nanxu (today Zhenjiang in Jiangsu), he was engaged in work at the office of the local governor.

His achievements included:

• the Daming calendar (大明曆) introduced by him in 465.
• distingushing the Sidereal Year and the Tropical Year, and he measured 45 years and 11 months per degree between those two, and today we know the difference is 70.7 years per degree.
• calculating one year as 365.24281481 days, which is very close to 365.24219878 days as we know today.
• calculating the number of overlaps between sun and moon as 27.21223, which is very close to 27.21222 as we know today; using this number he successfully predicted an eclipse four times during 23 years (from 436 to 459).
• calculating the Jupiter year is about 11.858 Earth year, which is very close to 11.862 as we know today.
• deriving two approximations of pi, which held as the most accurate approximation for π for over nine hundred years. His best approximation was between 3.1415926 and 3.1415927, with ³⁵⁵⁄₁₁₃ (密率, Milü, detailed approximation) and ²²⁄₇ (约率, Yuelü, rough approximation) being the other notable approximations. That is very good for that time, since he used sticks and cut them into a 12,288 sided polygon. No one discovered more of pi until 900 years later.
• finding the volume of a sphere as being 4πr³/3, where r is radius.

[edit] Named for him

• The lunar crater Tsu Chung-Chi

• 1888 Zu Chong-Zhi is the name of asteroid 1964 VO1.