Based on this fact, Zhang inferred other stars on the celestial bodies were also movable, overthrowing the conclusion reached by his predecessors. Halley (1656-1742), a British astronomer, also put forward a similar viewpoint, but was over a thousand years later. Zhang Sui is thus the first person to discover the movement of stars.

A scientist attaching huge importance to practice, Zhang's scientific methods were decisive to his great achievements. Together with Nan Gongshuo, he used a post to measure the shadow cast by the sun, and calculated the relations between the sun's position and the solar terms. Besides, Zhang also designed and made an astronomic instrument for measuring the altitude of the North Pole in different places throughout the country.

Between 724-725, Zhang led a large-scale project to identify the astro-geodetic measurement of 13 places and, based on the results, concluded that the length of a degree of the meridian line was 351.27 li (131.3 kilometers) by Tang measurement. This was the first measurement of the meridian ever done in the world.

Beginning in 725, Zhang set out to revise the calendar, and finished the draft, namely the "Da Yan Calendar,", before his death in 727. The new calendar was implemented in 728.

Well organized and logically deducted, the Da Yan Calendar was more precise than the previous ones, and was used for reference during the following dynasties. In 733, the calendar was introduced to Japan.

Author: Jessie