Monk Yi Xing: Astronomer, Mathematician, and Mechanical Engineer of the Tang Dynasty
Monk Yi Xing: Astronomer, Mathematician, and Mechanical Engineer of the Tang Dynasty

Monk Yi Xing was born in 683 with the given name Zhang Sui. His father had served as a magistrate for a few terms and died young. Left with no money and fatherless, Zhang Sui lived in poverty. Since his childhood, he had been keen on absorbing knowledge; he especially had an interest in calendar calculation and the Five Elements, as well as astronomy and calendar making.

At the age of 20, Zhang Sui went to Chang’an (the capital of the Tang Dynasty) in hopes of finding a master to help him further his studies. There he found a Taoist priest named Yin Chong who was a learned scholar.

Zhang Sui paid frequent visits to Yin Chong and borrowed books from him. One day Zhang Sui borrowed the book Tai Xuan Jing composed by the Confucian writer Yang Xiong from the Eastern Han Dynasty (25-220 CE). It was a complicated and obscure book about philosophy, nature and science. A few days later, Zhang Sui returned the book to Yin Chong. Yin Chong thought that Zhang Sui returned the book quickly because it was too difficult to understand and boring.

But when Zhang Sui showed him the notes he had jotted down to discuss with him, Yin Chong was surprised. Yin Chong had been studying the book for years but without a thorough understanding of it. Much to his surprise, Zhang Sui had complete grasp of the book. Since then, Zhang Sui had been hailed as an accomplished scholar.

Zhang Sui Becomes Monk Yi Xing

After Wu Zetian (the only female emperor in Chinese history) mounted the throne, her nephew was also given an important post.

Pursuing fame and praise, he sought to make friends with famous and learned scholars to elevate his social position. He declared that he would make friends with Zhang Sui.

As a person of integrity, Zhang Sui was unwilling to associate with such a vile person and refused his visits by giving excuses about being sick. However, the empress’s nephew tried all possible means to approach him. Unable to get rid of the disturbance, he thought of retreating into a temple as a Buddhist monk.

In 705, he left Chang’an and became a monk named Yi Xing in the Chongyang Temple in Song Mountain (Henan Province). He became a disciple of Master Pu Ji in religious cultivation. Zhang Sui’s given name gradually got lost and he became known as Monk Yi Xing thereafter.

As a monk, Yi Xing devoted his life to studying Buddhist scriptures, astronomy and mathematics. He also spent time traveling.

In 717, Yi Xing’s relatives recommended his services to Emperor Xuanzong. Upon learning of Yi Xing’s expertise in astronomy and mathematics, Emperor Xuanzong could not wait to see him. Very soon, Xuanzong arranged to have him work at Huayan Temple with monks from India to translate several Buddhist scriptures and to instruct the teachings of Vajrayana Buddhism. Emperor Xuanzong would often summon him to consult about stabilizing his country and nurturing his people. Yi Xing’s profound knowledge and realm earned himself significant trust from Taizong.

Monk Yi Xing Creates the Da Yan Calendar

In ancient China, farming was closely connected with calendar observations and seasonal changes. As a result, emperors of all dynasties put emphasis on astronomy.

Since 721, Yi Xing was made a court astronomer to engage in astronomic observation and calendar reform. Since the stored astronomical measurement instruments at the palace were too worn-out to be used, Yi Xing set out to design new instruments.

He first designed a model of the Tang armillary sphere, and asked the Emperor to have it cast in iron and bronze.

In 724, the Tang armillary sphere was successfully made to recalibrate the locations of 150 stars. The next year Yi Xing proceeded to create the structure of an elaborate water-powered armillary sphere, which reflected the regular motion of the sun, the moon and five major planets.
Additionally, it was an automatic hour-counter powered by water. The clock would hit a drum every Shi-chen (two hours) and strike every quarter Shi-chen. His invention was recognized as the first astronomical clock in China.

In 724, Yi Xing led a large-scale project to correct the calendar and to get an accurate measurement of the length of the meridian arc. Imperial assistant astronomers were dispatched to select observation sites (13 in all) from the plains in Northern China.

For example, at a Shadow Measuring Platform in Yangcheng, a two and a half meter tall stone gnomon was erected, replacing the platform, to make on-site measurements. After all measurements were complete, Yi Xing was able to accurately calculate the length of the meridian.

The creation of astronomic instruments and on-site observations were followed by the calendar revision in 724. He used features from the previous versions as reference to create a more precise calendar. With four years of hard work, the revised calendar named the Da Yan Calendar was published, and was comprised of 52 well-organized and logically-deducted volumes.
In 728, the Da Yan Calendar was implemented and received warm enthusiasm as by far the most accurate calendar.

The new calendar also spread to Japan and India. In 763, Emperor Junnin of Japan issued an order to abolish the then Japanese calendar and to implement China’s Da Yan Calendar instead. The Da Yan Calendar had been used for nearly 900 years from its publication in 727 up to the end of the Ming Dynasty when the western Gregorian calendar was introduced to China.

Yi Xing made three breakthroughs in the field of astronomy. He was the first to successfully measure the length of the meridian line. He was the first to come up with the theory that the distance between the moon and the sun was shorter than that between the earth and the sun. And he was the first to discover stellar motion.

Through observations, he discovered the regular motion of the sun and inferred that the other stars were also moving. Edmund Halley (1656-1742) also put forward a similar theory in 1718, but was over 1,000 years later. In 1982, the international community divided the moon into several regions, one of which was called “Yi Xing.”

Monk Yi Xing’s Passing

In 727, a man named Pei Kuan was appointed as the Henan governor. Devoted to his religious belief in Buddhism, he treated Monk Pu Ji as his master and paid daily visits to him. One day, Pei Kuan arrived at Monk Pu Ji’s place, and noticed Pu Ji was expecting an important guest. A moment later, a junior monk knocked on the door saying, “Monk Yi Xing just arrived.”

Yi Xing went into a house towards his master to pay his respects. Then Yi Xing had a very secret conversation with Pu Ji by whispering to him.

Pu Ji always responded by saying, “Yes. Yes.” After finishing, Yi Xing walked down the steps towards a house in the southern direction. Once inside, he locked the door. Then Pu Ji announced to his disciples: “Strike the clock! Monk Yi Xing has passed away!”

Pei Kuan and the monks rushed to the house where Monk Yi Xing was at and took a look inside. Yi Xing was sitting in meditation posture with eyes closed. When they tested his breathing, they found out Yi Xing had indeed passed away. Thus, like many accomplished monks in China’s history, Yi Xing knew that he had successfully completed his Buddhist cultivation and made arrangement for his “passing-away.”

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