Zhang Sui (also named Seng Yixing, 673-727), a monk of the Tang Dynasty (617-908), was a native of today's Nanle County in Central China's Henan 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.

After Wu 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.

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