Collected Illustrations of the Three Realms, Volume 4: Astronomy

Discussion on Eclipses of the Sun and Moon

In the section titled "Intersection of the Tenth Month" in the Classic of Poetry, it is recorded that "there was an eclipse of the Sun." original: "日有食之" (rì yǒu shí zhī). This refers to one of the earliest recorded solar eclipses in Chinese history, dated to 735 BCE. At the end and beginning of the lunar month, the Sun and Moon meet; if they share the same degree of longitude and follow the same latitudinal path, then the Moon covers the Sun, and for this reason, the Sun is eclipsed. The text describes a conjunction during a New Moon (晦朔), where the Moon passes directly between the Earth and the Sun. At the time of the Full Moon, when the Sun and Moon are in opposition, if they share the same degree and the same path, the Moon confronts the Sun and becomes eclipsed.

According to my assessment, the theory of the Moon covering the Sun causing a solar eclipse is easy to understand. However, the theory of the Moon confronting the Sun to cause a lunar eclipse is difficult to explain.

Earlier Confucian scholars claimed that the Sun’s essence is fundamentally Yin at its core. In this context, Yin represents darkness or opacity within the otherwise bright Yang essence of the Sun. Because it is Yin, there is an "obstructed spot" within it. At the Full Moon, when the two bodies are at opposite degrees and paths, the Moon and Sun confront one another. The Moon is then struck by the rays from that "obstructed spot" within the Sun, and thus it is eclipsed. original: "閼處" (è chù). This archaic theory suggested that a dark "core" of the Sun projected a shadow onto the Moon. This was the view of Master Hengqu Referring to the Neo-Confucian philosopher Zhang Zai (1020–1077 CE). and serves as the basis for the standard commentaries on the Classic of Poetry.

Yet, this theory is particularly dubious. The light of the Sun shines outward, leaving no place unilluminated. Even if there were darkness within its interior, that darkness would remain contained inside. How could it possibly project outward to "shoot" the Moon and cause it to lose its brightness?

Only the theory of Zhang Heng A polymath and astronomer of the Han Dynasty (78–139 CE) who correctly identified that the Moon is illuminated by the Sun and that lunar eclipses are caused by the Earth's shadow. seems easy to understand. Heng stated that the point directly opposite the Sun is a void as large as...