Xuanyeshuo 宣夜説

Xuanyeshuo 宣夜説 "Announcing the [phenomena of the] night" was a text on a cosmological theory founded by Xi Meng 郗萌 during the Later Han period 後漢 (25-220 CE). Of this book, only a few sentences have survived that are quoted in the treatise on astronomy (11-13 Tianwen zhi 天文志) in the history book Jinshu 晉書. Another collection of fragments is in Wang Renjun's 王仁俊 (1866-1913) Yuhan shanfang jiyu shu xubian 玉函山房輯佚書續編.

Of Xi's life, only his position as an assistant in the Palace Library (bishu lang 秘書郎) is attested. Liu Zhao's 劉昭 commentary on the astronomy chapter in the Xuhanshu 續漢書 (transmitted as part of the Houhanshu 後漢書, ch. 100-102) quotes two phrases on divination from the book. This suggests that Xi Meng was an astronomer-astrologer who specialised in celestial omens. However, his fact does not appear excessively superstitious or irrational, as the combination of the two disciplines was widespread, in China as in the West. The scholar Cai Yong 蔡邕 (132-192) explained that the Xuanye Theory had neither a teacher nor theoretical standards (jue wu shi fa 絶無師法).

It posits that the heavens are formless, intangible, and boundless, and that the vast sky appears as a hazy expanse solely due to human visual limitations. According to this theory, the sun, moon, planets, and stars naturally float in space, with their movements depending on qi 氣 ("breath, energy, air"), a fundamental force or ether-like substance. This explains why celestial bodies exhibit varied motions. Planets move in both direct and retrograde motion, whereas some stars remain fixed, like the North Star, while others, like the Big Dipper, do not rise and set in the same manner as the sun. The sun moves approximately one degree per day, while the moon moves thirteen degrees daily, indicating that their speeds are independent and unconstrained. The key argument of the theory posits that if celestial bodies were physically attached to a solid firmament, such variations in motion would not be feasible. Instead, their free-floating nature accounts for their diverse trajectories and behaviours.

The Xuanye Theory stands in opposition to two other ancient Chinese cosmological theories, namely the "Canopy-Heaven Model" (gaitian shuo 蓋天說), which depicted the sky as a dome covering the Earth, and the "Spherical-Heaven Model" (huntian shuo 渾天說), which described the heavens as a vast, rotating sphere surrounding the earth. Both theories were based on the existence of a tangible, physical celestial shell. In contrast, the Xuanye Theory challenged the existence of this firmament, offering a radically different perspective.

The Xuanye Theory was rooted in earlier Pre-Qin philosophical traditions, particularly the ideas of Zhuangzi 莊子 and the Song-Yin School 宋尹學派. The Zhuangzi (ch. Xiaoyaoyou 逍遙游) states: "Is the blue sky its true color? Or is it simply vast and boundless, with no ultimate limit? (qi yuan er wu suo zhi ji 其遠而無所至極)." This reflects Zhuang Zhou's 莊周 (trad. 369-286 BCE) speculation about the infinity of the universe. Song Xing 宋銒 (385-304) and Yin Wen 尹文 (360-280) proposed a rudimentary primordial theory of ether (mod. yuanqi xueshuo 元氣學說), attributing the origin of all things in the universe to qi. This qi could manifest in celestial bodies such as the sun, moon, and stars, as well as in earthly elements like mountains, rivers, grass, and trees. At the same time, Hui Shi 惠施 (ca. 370-310) of the School of Names (mingjia 名家) introduced a simple concept of infinity, stating: "That which is so vast that nothing lies beyond it is called the Great One; that which is so small that nothing exists within it is called the Small One." (zhi da wu dai, wei zhi dayi; zhi xiao wu nei, wei zhi xiaoyi 至大無外，謂之大一;至小無內，謂之小一).

These ideas laid the foundation for the Xuanye Theory's cosmological view of an infinite universe. The book Liezi 列子 (ch. Tianrui 天瑞篇) says, "The sun, moon, and constellations are simply luminous manifestations of accumulated ether (ji qi zhong zhi you guangyao zhe 積氣中之有光耀者). During the Three Empires period 三國 (220~280 CE), Yang Quan 楊泉 (d. after 280 CE) expanded, in his book Wulilun 物理論, on this idea, asserting that "Heaven is nothing more than primordial ether, vast and bright, containing nothing else" (tian yuan qi ye; haoran er yi, wu ta wu yan 天，元氣也，皓然而已，無他物焉). He also believed that the Milky Way was composed of ether, from which stars were born.

Other thinkers basing their theories on the Xuanye concept were Deng Mu 鄧牧 (1247-1306; in his Bo Ya qin 伯牙琴, ch. Chaoran guanji 超然觀記, Dong Gu 董榖 (in his Huanlongzi 豢龍子), Liu Zongyuan 柳宗元 (773-819) or Zhang Zai 張載 (1020-1077), who integrated the Xuanye Theory into their own philosophical and cosmological discussions.

In speculative natural philosophy, this hypothesis was remarkably insightful and unique. However, as a cosmological framework, the Xuanye Theory did not propose its own independent system for measuring celestial coordinates or the motion of heavenly bodies. Instead, it relied on the data from the Huntian Theory. This dependence was a key reason why the Xuanye Theory failed to achieve widespread development for over a thousand years.

The impact of Xuanye Theory suggests that, while it may not have been the dominant cosmological model of its time, it laid the groundwork for later theoretical advancements, subtly shaping Chinese scientific and philosophical thought. Although the Xuanye Theory failed as a working astronomical model, its greatest contribution lay in philosophy. Unlike the Huntian and Gaitian theories, which provided practical coordinate systems and measurement methods, the Xuanye Theory lacked empirical verification and could not effectively be applied to astronomical observations or calendar-making. Several unresolved questions further highlighted its limitations, mainly the idea of freely floating stars (still following fixed, predictable paths) or that stars not attached to the firmament will not fall down.