Zhang Sui (Seng Yixing)
Zhang Sui (also named Seng Yixing, 673-727), a monk of the
Dynasty (617-908), was a native of today's Nanle County in Central China's
Province. Bright and talented as a small boy, Zhang Sui read a lot of books
and worked hard at his studies. He went to Chang'an (the then capital of the
Tang Dynasty) to study astronomy and mathematics when he was a young man and,
after achieving enormous achievements, was hailed as a famous scholar.
Zetian (the only female emperor in Chinese history) mounted the throne, her
nephew was also given an important post. Angling for fame and praise, he sought
to make friends with famous and learned people to elevate himself. Zhang,
unwilling to associate with such a vile person, left Chang'an (modern-day Xian)
in anger and became a monk in Song Mountain.
In 712, when Emperor Xuanzong of the Tang Dyasty was enthroned, upon knowing
of Zhang's expertise in astronomy and mathematics, recalled him back to Chang'an
and made him a court astronomer. Zhang lived in Chang'an for another ten years,
and got the opportunity to engage in astronomic observation and calendar reform.
Soon after, Emperor Xuanzong mandated Zhang to preside over the calendar
revision. In order to measure the position of stars in their orbits and get the
law of movement, Zhang, in cooperation with Liang Lingzan, made the bronze
armillary sphere and ecliptic sphere. With the two instruments, Zhang carried
out effective astronomic research.
Prior to Zhang Sui, many astronomers, including Zhang
Heng of the Eastern Han
Dynasty (25-220), believed the stars were static. Zhang, however,
re-measured the position of over 150 stars and the degree between the 28
constellations and the celestial North Pole with the two instruments he
invented, hence concluding the stars were always in motion.