I-Hsing (683-727), born at Nanle County of Henan Province, was a great astronomer and Buddhist monk of the Tang Dynasty (618-907).

I-Hsing showed his intelligence from his childhood for his ability to remember whatever he saw. In his twenties, I-Hsing had read extensively and was well known for his mastery of the knowledge in the field of astronomy and mathematics. At that time, Empress Wu Zetian was an autocrat. She suspected righteous officials but wanted to recruit I-Hsing for some reason. Since he would not like to conform to Wu Zetian's reign, I-Hsing became a monk and was named I-Hsing when he was 24 or 25 years old. He studied Buddhist canons, astronomy and mathematics from then on so that his mastery on such knowledge was greatly improved.

After Empress Wu Zetian's abdication in 705, emperors of Tang wanted to recruit I-Hsing for many times while he refused. I-Hsing did not return to Chang'an (today's Xi'an City) until Li Longji's (Tang Emperor Xuanzong) invitation in 717.

I-Hsing received Tang Xuanzong's respect and trust so that he made progress in the fields of astronomic instrument manufacture, astrospace observations and geodesy. In 721, Emperor Xuanzong gave I-Hsing the job of revising the faulty calendar. I-Hsing studied former calendars and set up the Dayan Calendar on the basis of his accurate data on the movements of celestial bodies, which was his most important achievement. Textual researches showed that the Dayan Calendar was much more accurate and well organized than any other calendars at that time. Dayan Calendar was introduced to Japan in 733 and remained in use for almost one century. I-Hsing also built a new instrument, the Ecliptic Armilla, to observe the movements of the sun, the moon and five known planets. Furthermore, I-Hsing calculated the length of meridian degree for the first time in the world.

At the same time, I-Hsing translated many Indian Buddhist sutras. Combining Buddhism with the reality in India, he initiated Mahayana Buddhism different from Theravada Buddhism and became the head of Tantric Buddhism.