Denise Kaisler
kaisler@astro.ucla.edu
You know, the more I travel, the easier it is to see how ethnocentric our history books really are.
Take ancient astronomy for example. If you ask someone to list some famous astronomers of yore, you'll probably hear names like Aristarchus, Ptolemy, Copernicus, and Kepler. It's right and good that these names stick in our memories. They were brilliant men and deserve recognition. Yet there are many other astronomers who are not commonly known, merely because they didn't live in the culture that gave rise to ours.
A case in point is that of China. This nation has had a long history of astronomical successes. For example, in 1054 A.D., astronomers in the Middle Kingdom recorded the nova that gave rise to the Crab Nebula. They noted the brightness of this "new star" and how it glittered for several months before fading from view. Yet the European record does not mention this astronomical event at all.
Chinese astronomical records also indicate that their observers were making remarkable discoveries long before their European counterparts ever drew breath.
These ancient sages are known to have calculated the period of Halley's comet a full millennium before Halley himself saw the pattern, in a time when the gears of European science were slowed by religious persecution and the fall of Rome.
Yet during this dark time, on the other side of the world was well-lit -- a fact due in part to the astronomers of ancient China. Three men in particular are thought of as having made outstanding contributions. After reading about their lives, perhaps you will feel the same way.
Zhang Heng of the Eastern Han Dynasty
One of the premier intellects of the ancient world was undoubtedly Zhang Heng (78-139 A.D.). In his home country he is renowned for many things. First of all, he was one of Emperor An Ti's chief ministers. Beyond that, he excelled in writing and visual arts, in fact he is presently considered one of the four great painters of his era. To top it off, he was also the first man to invent seismograph and the odometer, or "mileage cart".
Yet Zhang's greatest contributions were in the field of astronomy. Intense powers of concentration allowed him to draw a detailed map of the heavens. His chart showed 124 constellations consisting of a total of 2500 stars, 320 of which were bright stars with names. "This is not including [those] observed by sailors," he wrote. "Of the small stars, there are eleven thousand five hundred and twenty."
However, Zhang was not satisfied with his paper maps of the sky. When his charts were complete, hired craftsmen to build what may have been the word's oldest three-dimensional models of the heavens. The first globes were made out of bamboo strips, but eventually he commissioned a bronze version that was almost five meters in circumference. This in itself would have been a singular accomplishment, but Zhang took it one step further. He used the power of water to make his globe complete one rotation every year, showing how the positions of the stars changed from one winter solstice to the next. Zhang Heng's invention is known as the water-powered celestial sphere.
By adding a few additional gears, Zhang was able to drive a pillar that demonstrated the waxing and waning of the moon. Remarkable, if one takes into account that Zhang Heng believed in an earth-centered universe. In one of his classic texts on the nature of the sky, he wrote that the heavens were like an egg with the Earth in place of the golden yolk.
However, there were two other cosmologies known to the ancient Chinese. One stated that the earth was a large square, 400 000 kilometers on a side and the sky was a spherical canopy that rotated counterclockwise while the sun and moon moved in the opposite direction. A third theory boldly asserted that the sky had no fixed shape and the celestial bodies merely floated around in it.
Zhang Heng did not subscribe to this last view. Instead, his cosmology was like the value of 3.1622 which he once obtained for pi -- a good approximation and remarkable for its time, but not entirely accurate. Of course, the same may one day be said of Einstein's work.
Zu Chongzhi, Astronomer and Mathematician
Another great scholar who was concerned with finding the value of pi was Zu Chongzhi (429-500 A.D.). From a very young age, he was interested in astronomy and mathematics. Legend has it that when he was ten years old, Zu accompanied his father on a trip to a construction site, which it was his father's job to oversee. As they returned from the trip, young Zu wondered why the moon was so much thinner than when they had set out two weeks ago. The elder Zu did not know the answer, but gave his son a book to read. It was an astronomy text written by none other than Zhang Heng.
Zu's father fostered his son's interest in the stars by apprenticing him to He Chengtian, a learned man of the time. While studying with Scholar He, Zu noticed a discrepancy between a sundial's shadow and the official calendar then in use. His master agreed that the calender had its problems and together they gathered enough data to correct them. He Chengtian's new system was called the Yuanjia calendar.
Yet even this calendar was not completely accurate. By the time Zu Chongzhi was a mature man, he noticed that it could not account for some of the solar eclipses so important to the imperial court.
Zu was successful in producing a new system, but, as so often happens in astronomy, the new idea got tangled up in red tape. Ministers of the imperial court spent two years arguing about whether they should use the new Daming calendar and might have gone on even longer if the emperor of the time hadn't passed away. With the death of the old emperor, discussion of what to do with the Daming calendar was postponed indefinitely.
But Zu Chongzhi wasn't disenheartened by this setback. Instead, he turned his attention to calculating a value for pi. Zu wanted to see if he could obtain a more accurate value than Zhang Heng had done.
To perform the calculation, Zu Chongzhi used a method of polygons. This geometrical scheme was strikingly similar to the argument Archimedes used to achieve his famous result 223/71 < pi < 22/7. It was unlikely that Zu knew of Archimedes' work. Still, Zu understood that the circumference of a hexagon was equal to exactly three times its diameter. But if he cut off the corners off the hexagon and made it into a dodecagon, that ratio became slightly larger than three. Zu Chongzhi painstakingly drew and measured a polygon with 192 sides, eventually finding pi to be equal to 355/113 or 3.1415929, a value which is correct to six decimal places. Such precision was not surpassed until until the 15th century A.D., when Al'Kashi, a native of Samarkand (now Uzbekistan), calculated pi using a similar method.
Although Zu Chongzhi's perserverence in deriving pi is exceptional, it is even more astonishing when one considers that his work predated even the abacus. Zu had to do all his calculations using nothing more than wooden counting sticks.
It was mainly for this contribution that Zu Chongzhi received a singular honor. He is one of the few Chinese mathematicians to have a lunar feature named after him. Crater Zu Chongzhi (listed in the IAU's handbook as Crater Tsu Chung Chi) is 28 km wide and is located about 20 degrees north of the moon's equator.
However, despite his mathematical victories, Zu Chongzhi never saw his Daming calendar put to use. It took years of effort by Zu's dutiful son before the calendar was at last recognized, a decade after his father's death.
The Brilliant Monk Yi Xing
Yet even though the Daming calendar was as accurate as Zu could manage, it too contained errors. Eventually it fell into disuse. Other calendars came and went, but they were little better than the one created by Zu.
By the year 721 A.D., the calendar was again in serious need of reform. The emperor at the time once waited an entire day for a solar eclipse so he could perform important rituals. When the predicted eclipse did not occur, the emperor was furious. The monarch promptly gave his advisor Yi Xing the job of revising the faulty calendar.
Yi Xing decided that he needed to take a detailed survey of the sky. However, the great bronze instruments used by previous court astronomers had been so neglected that they were no longer adequate. Yi enlisted the help of one Liang Lingzan to help build new ones. The learned men collaborated for four years and at last came up with a new type of instrument: the ecliptic armilla.
The device was cast in bronze. Its enormous weight was offset by carefully filling its various hollow parts with water. When perfectly balanced, the armilla was said to have responded to the touch of a finger. The device had several nested sets of circular rings. With them, Yi Xing and Liang Lingzan could determine the true solar time. They could also calculate the declinations of celestial objects and measure differences in right ascensions.
The two men also built a water-driven celestial sphere. It was based on Zhang Heng's design, but included separate tracks for a model sun and moon. These two orbs were calibrated so that they would circle the bronze globe once every 29 days and once every 365.25 days respectively.
With these instruments, Yi Xing was able to get more accurate data on the movements of celestial bodies. He finished the first draft of his new Dayan calendar just before his death in 727 A.D. The Dayan calendar was so accurate and well-organized that it remained in use until the sixteenth century A.D.
A century later, Jesuit missionaries brought European astronomy to China. For the first time in history, astronomers from the west and the far east began to exchange ideas. And as you'll see in next month's Event Horizon, it wasn't a one-sided flow of information.
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Notes on Pronunciation:
Perhaps another reason why we don't often talk about Chinese astronomers is that there names are so hard to say! Try wrapping your tongue around these three names using the guide below.
"Zhang Heng"
zh : like the j in "judge" but with the tongue curled towards the back of the mouth
a : like the a in "father"
e: like the u in "blur"
"Zu Chongzhi"
z: like the ds in "suds"
u: like the u in "flute"
ch: as in English, but with the tongue curled back
o: like the o in "or"
i: like the ee in "meet"
"Yi Xing"
x: like the sh in "shine"