Guo Shoujing, a famous astronomer, mathematician, and water expert of the Yuan Dynasty (1271-1368), was born in 1231 in Shunde Xingtai (today's Xingtai of North China's Hebei Province), and died in 1316.

At the age of 16, Guo began to intensely study astronomy, mathematics, and water conservation under the guidance of his forefather do you mean his grandfather or father?), accumulating rich knowledge in astronomy and geography, which was a touchstone for a good scientist in ancient China. Guo Shoujing scored great achievements in astronomical study, and the most notable of his accomplishments is his calendar -- the Shoushi Calendar.

In 1276, Kublai Khan, the first emperor of the Yuan Dynasty, captured the city of Lin'an (today's Hangzhou in East China's Zhejiang Province), capital of the Southern Song Dynasty (1127-1279). Although for many years the need for calendar reform had been understood, Kublai Khan saw political benefits in bringing in a new calendar at this time to emphasize how the new regime was replacing the old.

Guo, together with Wang Xun, were asked to set up a special bureau to undertake the necessary research and to make proposals for the new calendar, with Wang responsible for calculation and Guo for instrument building and observation.

In order to facilitate the development of the calendar, the Yuan Government set up the Observatory of Dadu (today's Beijing), the largest in the world of that time. Building began in March of that year and, following a design proposed by Guo, the work was completed in two months. Guo's friend Zhang Wenqian was appointed as director and Guo together with his colleague Wang Xun were the two co-directors. After four years' of strenuous efforts, the new calendar was finally finished, which was named the "Shoushi Calendar" by Kublai Khan.

Guo's calendar had 365.2425 days in a year, which was only 26 seconds different from the time it takes the earth to go around the sun. His achievement was 300 years earlier than the finalization of the modern calendar.

In the observatory Guo Shoujing invented, renovated, and improved 13 astronomical instruments. First he created a tower sundial, raising its height from the original 2.66 meters to 13.33 meters, which drastically increased its accuracy. Based on his research, the calendar was revised.

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