An Enterprise with Two Purposes: Measurement of Longitude in 17th and 18th Century China

By the end of the Ming dynasty, the Jesuit missionaries introduced the concept of geographical longitude to China. The measurement of the longitude was important to the Chinese in two respects: astronomically and geographically. The Chinese astronomers were obliged to predict ominous celestial phenomena such as solar and lunar eclipses. The measurement of longitude was necessary for predicting the exact moment of the eclipses at different place, especially for provincial capitals. Also it was realized that accurate measurement of longitude and latitude was essential for map making. From 1708 to 1707, the Kangxi Emperor commissioned Jesuit missionaries (most of them French) to survey the empire. The result was an unprecedented work The Complete Atlas the Qing Empire. I will investigate the methods used for measuring the longitude, and the accuracy of the data. I will also point out that the geodesic survey was not only a Chinese undertaking of calendar-making and the geodesic survey of the Qing Empire, but also a part of global endeavor for the measurement of the meridian, for the controversy over the shape of earth, etc. It is interesting to see how ideas and knowledge circulated between West and East in the 17th and 18th centuries.