Since time immemorial, humans have grouped the bright stars in the sky into imagined patterns, creating constellations. Every ethnicity and region has its own way of dividing constellations and their legends, usually naming them after people, animals, or objects. The 88 constellations currently used internationally are based on star charts that divide the sky into 88 different areas.
The Reconstruction of Sky’s Order
The story of constellation division starts after World War I. In July 1919, shortly after the smoke of war had cleared, European astronomers gathered in Brussels, Belgium, for the founding conference of the International Research Council. It was there they established a specialized astronomical management organization—the International Astronomical Union (IAU).
The IAU, formed from several organizations, is responsible for defining and naming newly discovered celestial bodies like stars, asteroids, satellites, and comets, as well as standardizing astronomical terms.
Starting in 1922, the IAU held a general assembly every three years to vote on resolutions and drafts. For instance, at the 26th General Assembly in 2006, astronomers officially reclassified Pluto as a dwarf planet, removing it from the list of planets. A century ago, another issue awaited this newborn institution: how to divide the sky into constellations and define their boundaries.
The Establishment and Standardization of 88 Constellations
Around 1900, most professional astronomers agreed that constellations had boundaries, but they couldn’t agree on the exact number of constellations or their precise boundaries. Despite German astronomer Johann Bode’s attempt to add boundary lines to his famous star chart "Uranographia," his proposal wasn’t widely accepted. From Bode’s time, cartographers would draw wavy lines between constellations on star charts, leading to inconsistencies and contradictions.
In May 1922, the IAU held its first general assembly in Rome, Italy, where astronomers decided to establish a commission to create international standards related to constellations. This Commission 3 was responsible for naming celestial bodies and laid the groundwork for the standardization of constellations. They approved a list of 88 constellations, which became the basis for the standardized division of the sky.
Interestingly, while they established 88 main constellations, they actually published 89 names, including the ancient Greek constellation "Argo." However, due to its large area, it was later divided into three constellations: Carina, Puppis, and Vela.
The Treaty of Sky’s Division
After standardizing constellation names, the next step was to define the areas of each constellation, much like surveying and delineating the borders between countries. In 1925, at the second IAU meeting in Cambridge, England, Belgian astronomer Eugène Joseph Delporte submitted a proposal for defining constellation boundaries. His proposal was quickly accepted, and he spent the next two years searching for feasible boundary schemes.
At the time, the IAU was filled with various nationalistic sentiments, especially from countries like France and Germany, which made it challenging to resolve disputes. Delporte adopted a cautious approach, negotiating with committee members to reduce resistance in the process of establishing constellation boundaries.
One of Delporte’s guiding principles was to keep all named variable stars within their existing constellations. To meet these conditions, he decided to continue using some previously adopted boundary schemes. For instance, to align with American astronomer Benjamin Apthorp Gould’s early work, he largely retained the southern constellation boundaries from Gould’s "Uranometria Argentina," published in 1879, making only appropriate adjustments.
Delporte focused on the area north of 12 degrees south of the celestial equator, supplementing the work of Gould and others. He drew the latest constellation boundaries along the right ascension and declination lines, ensuring each star was in a specific constellation. For example, the star shared by Auriga and Taurus was finally assigned entirely to Taurus as Taurus β.
Another challenge Delporte faced was dealing with the constellation Ophiuchus and Serpens, which have been intertwined since ancient times. Delporte split Serpens into two parts, the head and tail, placing them on either side of Ophiuchus, solving this issue. Today, Serpens is the only constellation split in two, though both parts are still considered the same constellation.
In 1928, the IAU’s third meeting took place in Leiden, the Netherlands, where astronomers officially approved Delporte’s new constellation boundaries. In the spring of 1930, the IAU discussed modifications to the proposal, and the new constellation boundary scheme was finally approved. Subsequently, Delporte published the latest "Celestial Atlas" in April 1930, accompanied by the explanatory book "Scientific Definition of Constellations." From then on, the sky, like countries on Earth, had clear "division treaties," which all astronomers have adhered to ever since.
This article is from "Science World" magazine, Issue 11, 2021.
Author | Li Liang (Institute for the History of Natural Sciences, Chinese Academy of Sciences)
New Media Editor | Zhang Lijun
Proofreader | Fang Chen
