Orion constellation - Photo by john ko on Unsplash

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Contents

• Why Are There Constellations?
• IAU and the 88 Constellations
• Constellation Figures
• Constellation Names
• Charts
• More Information
• Acknowledgements
• References

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Why Are There Constellations?

Ever since people first wandered the Earth, great significance has been given to objects seen in the sky. Ancient peoples noticed that the rising and setting points of the Sun and the Moon, as well as the visibility of stars, change throughout the year. Therefore, all cultures have long traditions of using astronomy as a tool of survival. They developed astronomical methods for navigation in space and time, in particular, to determine their calendars: to measure equinoxes and solstices and to predict the seasonal behaviour of nature. For the Inuit in the far north, this was the prediction of the return of the Sun; for the Egyptians, it was the flood of the Nile; for the Babylonians and others in the monsoon zone, it was the predominant direction of the wind; and for the Aboriginal Australians, it was the emu’s breeding cycle. 

How Humans Made Constellations

The images interpreted by cultural groups from patterns of stars depend on the natural environment and cultural habits: only people who have seen emus can interpret such a bird in an arbitrary pattern in the sky. The effect of human cognition that makes us see patterns in arbitrary configurations (like clouds, mountain chains, or stars) is studied by gestalt psychology. Several network analysis studies have shown that the stellar patterns were grouped in similar ways all over the Earth (Bucur 2021, 2022; Kemp et al., 2022). However, humans are also social animals and love to exchange. As we invented languages and symbols, we learned to exchange knowledge and communicate with others of our species. Observations of the skyscape and celestial patterns have always been exchanged among people of various cultures.

Practical Application

In particular, the Chinese and the Inca both grouped their constellations according to the four seasons. In the case of the Inca, each season was accompanied by “dark constellations”: constellations formed from the dark clouds in the Milky Way, not from stars (Gullberg et al., 2020; e.g., Figure 1). The animals seen in this procession indicate events in nature: when the constellation of the “Llama nursing its offspring” appears, llama babies are born, when Fox appears, fox babies are born, when then the constellations of the Snake and the Toad appear, it is the wet season, and so on.

The Dark Constellations of the Inca – right to left: (1) Machacuay, (2) Hanp’atu, (3) Yutu, (4) Yacana, (5) Uñallamacha, (6) Atoq, and (7) Michij runa.
Art by Jessica Gullberg, constellations inspired by Gary Urton and Miguel Araoz Cartagena. Image Credit: Gullberg et al. (2020, fig. 4), reworked by Gullberg and Gullberg (2022).

The ancient Chinese constructed tiny constellations, some consisting of only one to three stars. However, these small constellations were grouped into super-constellations, covering extended areas and dividing the sky roughly into quarters. Their names are not similarly directly linked to nature as the Inca constellations, though they are also rooted in nature-based belief systems.

Some of the common Chinese asterisms (yellow lines) are grouped into the super-constellations Dark Warrior (winter), Azure Dragon (spring), Vermilion Bird (summer), and White Tiger (autumn). Here they are contrasted with the modern European asterisms (cyan) of the Autumn Square, the Summer Triangle, the Spring Triangle and the Winter Hexagon. Image credit: Susanne M Hoffmann.

The 88 IAU Constellations

By the 19th century, over a hundred constellations could be found on various star charts since there was nothing to stop astronomers from inventing new figures between the existing ones, which they often did to flatter a patron or monarch. In the 19th century, there were several changes in the scientific method and research focus. Many variable stars were discovered, stellar transients were investigated, and astrophysics with electric measurements of brightnesses and spectra of stars was developed. With the high number of investigated stars, new nomenclatures emerged, many of them relying on the constellation names. Therefore, one of the IAU’s first tasks after its formation was to define an internationally agreed set of constellations. At its first General Assembly held in Rome in 1922, the IAU’s Commission on Notations and Units agreed on a list of 88 constellations covering the entire sky, with three-letter abbreviations of their Latin names. These are the 88 IAU constellations that are still recognised today, as described in more detail below.

But these constellations still lacked officially defined boundaries, and this was the next task to be tackled. At the second IAU General Assembly held in Cambridge, England, in 1925, Eugène Delporte of the Royal Observatory in Brussels presented proposals for a clearly defined system of constellation boundaries drawn along lines of right ascension and declination, the celestial equivalents of longitude and latitude on Earth. The IAU took up the suggestion and appointed Delporte to prepare official boundaries with the specific requirement that all known variable-brightness stars should remain within the constellation to which they had already been assigned. This resulted in some awkward-looking outlines.

Front cover of the IAU book edition: Delporte’s Délimitation scientifique des constellations. Image Credit: Michael E. Bakich library

Delporte’s new boundaries were approved by the IAU at its General Assembly at Leiden in 1928, and the results were published in 1930 in Délimitation Scientifique des Constellations along with an accompanying Atlas Céleste. So constellations, which started out as imaginary patterns of stars, became scientifically defined areas of sky. Nowadays, when astronomers say something is “in” a given constellation, they mean it is simply within those defined boundaries, not that it is part of any star pattern.

Constellation Figures

In contrast to the historical and Indigenous concepts of constellations as patterns of light points in the night sky, such as the Chinese star chains or the lists of stars under a common headline in historical star catalogues, the IAU defines a constellation as an area in the sky surrounded by its boundary. The boundaries indicated by sky coordinates are the defining quantity. Note that these boundaries, though the standard recognised by the IAU, are not necessarily universally utilised: many cultures have had their own asterisms for hundreds or thousands of years and continue to use them.

In star maps or charts, it is common to mark line patterns representing the shapes that give the name to the constellations. For IAU constellations, no stick figures or figurative painting styles are formally defined. Consequently, the same constellation may have several variants in its visual representation.

In addition to the constellation line patterns, there are also asterism line patterns. Asterisms are distinct from constellations. They are patterns or shapes of stars that are not related to the known constellations but are widely recognised. Examples include: the seven bright stars in Ursa Major, known as “the Plough” in Europe, “the Big Dipper” in America and “the Northern Dipper” traditionally in China; and the “Summer Triangle”, a large triangle seen in the summer and autumn night sky of the northern hemisphere, composed of the bright stars (from three different constellations) Altair, Deneb and Vega. Asterisms can even consist of star clusters (like “the Seven Sisters”, or the Pleiades cluster), groups of star clusters (like “the Golden Gate of the Ecliptic” consisting of the Pleiades and Hyades clusters) or of only one bright star (like the Greek star Capella or the Chinese single star-asterisms). 

It should be realised that the groups of stars that made up the original constellation patterns have, in most cases, no physical connection with each other. All constellations are a matter of perspective. They are simply our Earth-based interpretation of two-dimensional star patterns on the celestial sphere, composed of stars of various different brightnesses and at widely differing distances from Earth.

Constellation Names

Each Latin constellation name has two forms: the nominative, used when talking about the constellation itself, and the genitive, or possessive, used in star names. For instance, Hamal, the brightest star in the constellation Aries (nominative form), is also called Alpha Arietis (genitive form), meaning literally “the alpha of Aries”.

The table below includes the Latin names for each IAU constellation, their abbreviated names and a chart showing their boundaries. These are a mix of the ancient Greek patterns recorded by Ptolemy in addition to some more “modern” patterns observed later by astronomers.

Pronunciation of Constellation Names

Experienced astronomers, both professional and amateur, may pronounce constellation names differently but have no trouble understanding each other. There is no single correct way to pronounce a constellation name, and several sources address the issue. See, for example:

• Constellations Names and Pronunciation, Sky & Telescope (2007)
• Latin Pronunciation Demystified, Michael A. Covington (2010)
• Star Name Pronunciation, H. B. Rumrill (1936) 

Charts

The charts below were produced in collaboration with Sky & Telescope magazine (Roger Sinnott and Rick Fienberg). Alan MacRobert's constellation patterns, drawn in green on the charts, were influenced by those of H. A. Rey but, in many cases, were adjusted to preserve earlier traditions. Each chart was produced using the J2000 epoch. The images are released under the Creative Commons Attribution 4.0 International license.

Charts Graphical Legend

Charts Text Legend

Each constellation comes with the following information:

• Name: The name is the Latin name adopted by the International Astronomical Union in 1930.
• Pronunciation of the name (as described above)
• Abbreviation: the standard three-letter form of the Latin name
• English Name: The popular name in English
• Genitive: The genitive is the possessive form of the constellation’s name in Latin. For example, alpha Orionis is the alpha star in the constellation of Orion.
• Pronunciation of the genitive (as described above)
• Chart for screen view (GIF): Chart in GIF format, 1000 pixels wide
• Chart for printing (PDF in A4 format): Chart in PDF format, to be printed in A4 format
• Boundary coordinates (TXT): A text file containing a set of coordinates that defines the boundaries of the constellations in the sky. The format is: HH MM SS.SSSS| DD.DDDDDDD|XXX
  • HH MM SS.SSSS defines the right ascension hour, minute and second with J2000 coordinates
  • DD.DDDDDDD defines the declination with J2000 coordinates
  • XXX is the abbreviation of the constellation name
  • | is the separator of the fields
  • Example: 22 57 51.6729| 35.1682358|AND

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Charts

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More Information

• Report from the 1922 IAU General Assembly

Acknowledgements

We would like to acknowledge the contributions of WGSN President, Susanne M. Hoffmann, IAU OAO Director, Kelly Blumenthal, WGSN Vice President, Eric Mamajek, and WGSN members Robert H. van Gent, Ian Ridpath and Javier Mejuto (Honduras National Outreach Coordinator), former IAU Director of Communications, Lars Lindberg Christensen (originator of the first version of this Theme), and former Head of the IAU web team, Raquel Yumi Shida.

References

• Bucur, D. (2022). The network signature of constellation line figures. PLOS ONE, 17(7).
• Covington, M. A. (2002). Celestial Objects for Modern Telescopes. Cambridge University Press, pp. 80-84.
• Davenhall, A. C.; Leggett, S. K. (1997). Constellation Boundary Data (Davenhall+ 1989). VizieR On-line Data Catalog: VI/49.
• Debarbat, S. (1988). Mapping the sky: past heritage and future directions: proceedings of the 133rd Symposium of the International Astronomical Union, held in Paris, France, 1-5 June 1987. International Astronomical Union. Symposium no. 133, Kluwer Academic Publishers, Dordrecht.
• Delporte, E. (1930) Délimitation scientifique des constellations. Cambridge University Press.
• Flanders, T. Constellation Names and Abbreviations. Sky & Telescope.
• Gullberg, Steven R, Hambacher, Duane W., Martín-Lopez, Alejandro, Mejuto, Javier, Munro, Andre M. and Orchiston, Wayne (2020). A cultural comparison of the “Dark Constellations” in the Milky Way. Journal of Astronomical History and Heritage, 23(2), 390‒404.
• Hoffmann, Susanne M and Wolfschmidt, Gudrun [ed.] (2022). Astronomy in Culture - Cultures of Astronomy, Nuncius Hamburgensis, Vol. 57, Hamburg/ Berlin.
• Kanas, Nick (2007, 2012, 2019). Star Maps - History, Artistry, and Cartography, Springer Nature, Cham Kemp, Charles, Hamacher, Duane W., Little, Daniel R., Cropper, Simon J. (2022). Perceptual Grouping Explains Similarities in Constellations Across Cultures, Psychological Science Vol 33, Issue 3.
• Ridpath, Ian, The IAU list of the 88 constellations and their abbreviations
• Shitong, Y. (1981). Chinese and Western Contrast Star Chart and Catalogue 1950.0 (Star Catalogue). Science Press. [伊世同《中西对照恒星图表1950.0(星表分册)》（科学出版社 1981)].