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A Great Chinese Astronomer
By David James
9/2/2005



Zhang Heng was a famous Chinese scholar and astronomer from the time of China’s Han Dynasty. The Han era was regarded as one of the most prosperous in Chinese history, in which rapid advances were made in science, astronomy, technology, medicine and the arts. Many scholars made great contributions to the various scientific fields, Zhang Heng being one of the more remarkable.

Zhang Heng was born in 78 AD at Shiqiao town, located 25km north of the present Nanyang city, Henan province. He was most recognised for his contributions to astronomy and other sciences while he was in service for the government. His roles included observing astronomical phenomena, compiling calendars, forecasting weather and arranging meteorological studies.

Zhang Heng’s most recognised contributions were to the field of ancient Chinese astronomy. He authored many books in this area, the most famous titled Lingxian. In Lingxian, he proposed many concepts that were advanced for his time, such as the moon itself does not emit light but rather reflects sunlight. During the day, sunlight overpowered the light of the moon, so the moon was not visible.

Zhang Heng believed that when the moon was full it should be visible to the eye, and the phases of the moon were due to the earth covering the sunlight.

Mr. Zhang also explained the reason behind the lunar eclipse. He also proposed that when the moon passes the shadow of the earth, a lunar eclipse would occur. Additionally, in Lingxian, Zhang Heng produced calculations for the angular diameter of the sun and the moon. He recorded 2,500 stars that he observed while in Luoyang.

Zhang Heng also created some remarkable inventions. He designed and built the world’s first “Armillary Sphere,” an instrument used for astronomical calculations. In one of his publications titled “The Chart and Interpretation of the Armillary Sphere”, he measured one solar year as “365 and a quarter” days, a figure remarkably close to modern astronomers’ estimate of 365 days, 5 hours, 48 minutes and 46 seconds.

In 132AD Zhang Heng invented the world’s first seismograph able to measure the direction of an earthquake, 1700 years before the first European seismograph. He also invented an odometer, a sundial and a wooden bird that could fly.

Although Zhang Heng lived almost two thousand years ago, his achievements show human wisdom and ingenuity have always existed, and our longing to understand the universe has not changed.

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