Two Epistemologies of the Sky

This article is part of the Map Chat series commissioned by Richard Pegg, Director and Curator of the MacLean Collection in Illinois.

In 1687, in the workshops of the Korean Joseon court, a chart of the heavens was carved into stone, and from that stone a rubbing was taken in ink on paper. One such rubbing, reproduced here, belongs to the MacLean Collection. At its center is a circular planisphere of the northern sky: well over a thousand stars, marked as small discs and linked by fine lines into hundreds of named constellations, the Milky Way traced across them as a pale band, the whole disc divided into wedge-shaped sectors by radial lines running from center to rim.

Dense columns of text frame the planisphere above and below. The title runs across the top of the sheet: 天象列次分野之圖, Cheonsang yeolcha bunya jido, “Chart of the Positions of the Heavenly Bodies in Their Natural Order and Their Allocated Fields.” That final phrase—"allocated fields"—is the key to everything this object is and does.

The 1687 stele was itself a re-carving. The original had been engraved in 1395, only three years after the founding of the Joseon dynasty, on the order of its founding king, Taejo. It belonged, in turn, to a far older East Asian tradition of monumental star charts, whose most celebrated monument is the Chinese Tianwen tu (天文圖), designed by the Song scholar Huang Shang and engraved in stone at Suzhou in 1247. The Korean chart stands squarely in that lineage. To read the MacLean rubbing closely, then, is to read an object that gathers up five centuries of a particular way of thinking about the sky.

And it is a very particular way. Set this chart beside the other great pre-modern tradition of celestial mapping—the Greek tradition codified in Ptolemy’s Almagest around 150 CE—and the natural reflex is to compare them as two attempts at one task, and then to keep score: who catalogued more stars, whose measurements were more accurate, whose projection was more mathematically powerful? That comparison is not wrong, but it is shallow. The deeper point is that the two traditions were not answering the same question. They were not two stages on a single road toward modern astronomy. They were two different skies.

The geometric sky

For Ptolemy, the heavens were a geometric model. The purpose of the Almagest was prediction: to determine where the Sun, Moon, and five planets would stand at any future moment, with enough precision that the prediction could be tested against observation (fig. 3).

Its catalogue of 1,022 stars, grouped into 48 constellations, was the fixed reference grid against which those wandering bodies could be tracked. The constellations themselves were pictorial figures borrowed from Greek myth (like Orion the Hunter, the Great Bear, or the Scorpion) and a star mattered chiefly insofar as it belonged to one of these recognizable figures. Stars that fit no figure were catalogued under a revealing label: amorphōtoi, “unformed”

The framework rested on a metaphysics. For the Aristotelian world Ptolemy worked within, the heavens were a separate and superior realm, composed of an incorruptible fifth element and turning in perfect, uniform circles. They obeyed their own laws. They did not watch the earth and did not comment on it. They simply were — and the astronomer’s task was to recover, through geometry and patient observation, the mathematics of their indifference.

The governed sky

The sky of the Korean chart is not indifferent at all. Here heaven and earth are not two separate realms but two poles of a single, continuously interacting order—and the structure of the chart says so at every level. At its center sits the north celestial pole, the still point around which all the stars wheel; and the pole, in this cosmology, is the celestial counterpart of the king.

The comparison was canonical: the ruler who governs by virtue, says the Analects, is “like the North Star, which keeps its place while all the stars turn toward it.” To set the pole at the center of the chart was to map the sky as a governed order.

The constellations confirm it. Where the Greek sky is populated by hunters and beasts, the East Asian sky mapped here is populated by an administration. Its highest tier consists of three “enclosures”: the Purple Forbidden Enclosure around the pole, answering to the royal palace itself; the Supreme Palace Enclosure, answering to the civil government; and the Heavenly Market Enclosure, answering to the merchant quarters. Beyond them, the twenty-eight “lunar mansions”—the wedge-shaped sectors so plainly visible on the rubbing—divide the band of sky through which the Moon travels. This is not a gallery of myths. It is a mirror of the state.

Here the “allocated fields” of the title become decisive. The outer rings of the chart bind each sector of the heavens to a specific territory on the earth below—the punya 分野 (Chinese fenye) system, in which the sky is laid directly over the map of the realm (fig. 5).

The consequence is immediate and practical. Whatever appears in a given sector of the sky—a comet, a “guest star” where none had been, an unexpected meeting of planets—carries meaning for the corresponding region on the ground. The chart is therefore not only a scholarly reference; it is an instrument of governance, a surveillance map by which a court, in watching the heavens, simultaneously watched the political health of every province.

Behind all of this lay the doctrine of the Mandate of Heaven: the principle that a dynasty’s legitimacy was continuously sanctioned, monitored, and, should rule fail, withdrawn by heaven itself. Irregularities in the sky were messages, and the royal observatory existed to read them. This is also why such charts were carved in stone. Stone is the medium of permanence and public authority; to engrave the sky was to assert that this order, and this correspondence between heaven and earth, was fixed—and that the dynasty maintaining it was fixed along with it. King Taejo’s stele of 1395 was, in precisely this sense, a founding document of the new Joseon state.

Two skies, not one

The contrast, then, is not finally technical but ontological. The Ptolemaic astronomer asked of a planet: where will it be, and how can I compute that? The Korean court astronomer asked of the same point of light: what does its position mean, and for whom? One tradition built a mathematics of the heavens’ independence; the other built an infrastructure for reading the heavens’ judgement.

Neither was a failed version of the other. When European Jesuit astronomers reached the Qing court in the seventeenth century, their more accurate eclipse predictions did not overturn the governed sky: the Kangxi emperor simply absorbed the new mathematics as a sharper tool for the old purpose of reading heaven’s messages. And the equatorial framework of the East Asian charts—with pole at the center—turned out, ironically, to be structurally closer to the coordinate system modern astronomy eventually adopted than Ptolemy’s ecliptic ever was. Technical success and underlying worldview are not the same thing.

This is the quiet challenge the MacLean rubbing poses to anyone tempted to file it simply under “early astronomy.” Two charts can look profoundly alike—both circular planispheres, both mapping hundreds of stars, both carrying a coordinate grid—and still rest on foundations so different that the very word “comparison” must be handled with care. The 1687 Korean chart and Ptolemy’s Almagest do not map the same sky. They map two different skies, made by two different ways of asking what a sky is for.

The chart in the MacLean Collection has long outlasted the court that commissioned it and the cosmology that once made it fully legible. That it still draws us to look — still poses its question — is its own kind of argument. The governance of the heavens was, for those who mapped them, a matter serious enough to commit to stone.

Further reading

Richard A. Pegg, “Celestial Chart,” in Heaven and Earth: The Blue Maps of China (Boston: Leventhal Map & Education Center at the Boston Public Library, 2024).

Richard A. Pegg, “A Korean Star Chart Screen from the Mid-18th Century,” Orientations 55, no. 6 (November/December 2024): 48–54.

Joseph Needham, Gwei-Djen Lu, John H. Combridge, and John S. Major, The Hall of Heavenly Records: Korean Astronomical Instruments and Clocks, 1380–1780 (Cambridge: Cambridge University Press, 2004).

David W. Pankenier, Astrology and Cosmology in Early China: Conforming Earth to Heaven (Cambridge: Cambridge University Press, 2013).

Salvatore Martinelli is currently Director of the Harmonikamuseum in Trossingen, Germany. He was a MacLean Collection Map Fellow in 2023.