Opticien célèbre. Zhang Heng 张衡

Astronome, mathématicien, ingénieur et poète chinois, Zhang Heng a vécu pendant la dynastie Han qui régna sur la Chine de 206 avant J.-C. à 220 après J.-C. Il est connu pour sa connaissance étendue des engrenages et de la mécanique, et pour avoir inventé le premier sismographe, ainsi que la sphère armillaire à énergie hydraulique pour représenter les observations astronomiques. Il a également perfectionné la clepsydre, et amélioré les calculs chinois de la formule du nombre π.

Restoration and re-creation in the Eastern Han

We look first at the situation in the early years of the restored Han dynasty. Liu Xin’s system continued in use for more than half a century. Then, in 85 CE, Liu Xin’s system was replaced. We have records of the practical and theoretical grounds on which the old system was rejected, and of the creation and implementation of a new system. Next we follow the story of how c. 92 CE Jia Kui advocated a fundamental innovation in both theory and practice: he insisted on the ecliptic as being central to astronomical observation and calculation. The richness of records from this period makes it easy to tell a detailed story of technical innovation in its fullest context, leading up to the work of Zhang Heng (78–139 CE), for whom astronomical calculation was just one of several fields in which he gained a reputation for exceptional originality.

Zhang Heng's Huntian yi zhu Revisited

The Huntian yi zhu is one of the earliest surviving works on astronomy in ancient China. It is traditionally attributed to Zhang Heng (78-139) of the Eastern Han. Modern scholars have studied the text for the past seven decades and there seems to be agreement on its interpretation, especially on its mathematical contents. This article identifies one major flaw in the assumptions among the modern studies of the piece: the term shao ban, incorrectly taken to mean “one quarter,” should mean “one third.” The mathematical interpretation of the Huntian yi zhu by scholars including Christopher Cullen and Chen Meidong has to be reworked. A new reading is presented here, which demonstrates Zhang Heng's meticulousness on the one hand and our adherence to the philological principle of lectio difficilior potior (the more difficult reading is the stronger) on the other.