Zhang Heng

78-139

Chinese Astronomer and Mathematician

Zhang Heng was a Chinese mathematician and scientist, born in the first century a.d.In addition to being one of the emperor's astrologers, Heng made a number of interesting astronomical discoveries. However, his most important invention was the seismograph, used to detect earthquakes at a great distance.

Virtually nothing is known of Heng's early life or, indeed, of his nonprofessional life. What is known is that he was a multitalented scientist who became a minister in the Chinese government during the rule of Emperor An'ti. During this time, he was the chief astrologer as well as a respected mathematician and scientist.

Heng is known to have made a number of important contributions during his life. As an astronomer, he made a very good attempt to bring some order to the stars, describing the sky as being composed of "124 groups which are always brightly shining. Three hundred twenty stars can be named. There are in all 2,500, not including those which the sailors observe. Of the very small stars there are 11,520." Presumably, Heng's reference to the sailors is acknowledging the fact that the skies at sea are darker and more clear than those ashore, allowing more stars to be seen.

Another of Heng's accomplishments was that, in a.d. 123, he revised the Chinese calendar in an effort to bring agreement between the calendar and the seasons. This is similar to corrections made in Europe, including the introduction of the leap year, and the addition of 11 days to the calendar during the eighteenth century. Heng's solution was not a permanent fix, but it did serve its purpose in reconciling the calendar and seasons for some time.

It is likely, however, that Heng's most significant contribution was the seismograph. Today, seismographs are extraordinarily sensitive instruments that not only alert us of earthquakes anywhere on Earth, but also serve notice of nuclear weapons testing. Tracking the reverberations of earthquakes, too, has given scientists an unparalleled tool to look inside Earth, tracking how the waves from earthquakes propagate through the planet and what this tells us about Earth's interior.

Heng's instrument was not, of course, nearly so sophisticated. Nonetheless, it was a significant accomplishment that, in and of itself, was sufficient to assure his reputation as a scientist. What most amazed his contemporaries were those instances in which an earthquake was detected so far away that a messenger would take several days to reach the imperial capital with the news. His instrument consisted of four dragon's heads, each holding a ball. When an earthquake struck, the ball was knocked loose, falling into a metal bowl. The direction in which the earthquake occurred could also be determined by seeing which ball of the four had fallen.

In spite of his relatively long and productive life, little else is known of Heng than this. However, his invention of the seismograph alone would be sufficient to assure him a place in the annals of science. Heng died seven years after first demonstrating his seismograph, at the age of 61.

P. ANDREW KARAM