The Lunar Apogee and Nodes in Premodern East Asian Astronomy

• In Event: New Approaches to the History of Science in Premodern and Imperial China

English Abstract

Ancient Chinese astronomy initially viewed the Moon’s orbit differently from Greek astronomy, which saw it intersecting with the ecliptic. In China, the concepts of the lunar apogee and perigee—the Moon’s farthest and closest points to Earth—remained vague until around 718 CE, when Indian mathematical astronomy, introduced through a translation, first clearly defined these ideas for Chinese astronomers, although they were not widely studied. About a century later, these concepts were reintroduced in a more accessible way and eventually woven into the emerging practice of horoscopy that had been introduced from abroad. Like Indian astronomy, Chinese astronomers treated Rāhu and Ketu—the lunar nodes, or “Head and Tail”—as pseudo-planets, i.e., as moving points along the ecliptic. Uniquely, though, the Chinese also used the lunar apogee in a similar role. Greek horoscopes noted the apogee but did not emphasize its importance, unlike their Chinese counterparts. Understanding the apogee and nodes was crucial for predicting eclipses and their magnitude, but East Asian astronomers remarkably attempted this using a flat-earth model. In contrast, Greek and Indian astronomers, working with a spherical Earth, understood the reality of the Earth’s shadow. Chinese astronomers, with their distinct cosmology, had a different explanation for the dark mass obscuring the Moon during a lunar eclipse. Japanese astronomy fully adopted this Chinese system. This paper explores the history of the lunar apogee and nodes in East Asian astronomy, highlighting in Chinese and Japanese contexts the challenges posed by a flat-earth cosmology.

Author

• Jeffrey Kotyk, Max Planck Institute for the History of Science