What is a Constellation?

by Tammy Plotner on August 25, 2009

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Since humankind first began recognizing patterns in the stars, we sought to give them recognizable meaning by connecting the brighter ones in a dot-to-dot fashion and assigning them characters of myth and legend. Be it animal, spiritual or complete fancy – these very first labels put on stellar patterns were the beginnings of the constellations.

The First Constellations

Stellar patterns are known as asterisms – an association of stars based on what we see, not a true physical relationship. The first constellations were based on asterisms, groups of stars that appeared to resemble something and in a position in the sky which gave them meaning. In this case, the signs of the Zodiac – or zodiacal constellations – were quite probably among the first to be recognized inter-culturally. Although the stars might be hundreds of light years apart and their relationship to the Sun, Moon and planet Earth thousands of light years further, each civilization recognized these particular patterns of stars fell into the imaginary path the Sun and Moon took across the sky known as the ecliptic plane. Even before humans recognized there were other planets, we knew and understood that at times certain areas of the sky could contain extra “stars” – so we sought to express them.

zodiacWhile traditionally we know the zodiacal constellations as Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces they shared many names in various cultures and religions – often meaning much the same thing. For example, the Hindu culture refers to Aries as Mesa “the Ram” and Gemini as Mithuna “the Twins”. Placing definitive names on the constellations of the zodiac may have come about as early as 2700 B.C. as the need began to arise for our first primitive coordinate systems. While each culture took their coordinate systems slightly differently, for example the Greeks chose tropical time, while the Hindu and Chinese based theirs on sidereal time – the fact remained these asterisms were soon to become “universally” known.

With time, other nearby asterisms soon began to take shape and became constellations of their own as well – each with names and cultural significance. However, there was only one problem. These first constellations had no definition – no borders – no consistency. As the Earth’s first navigators began to travel, they began to add more and more stars to their charts, comparing them to known charts of the time, and the whole issue was becoming very confusing. It would be very hard to navigate towards a star you couldn’t see until you got there – let alone a constellation that wasn’t even charted on your maps!

The First Official Constellations

PtolemaeusBy the 2nd century AD, the Greek astronomer Ptolemy decided it was time to start getting serious about unifying these asterisms and assigning them constellations names. His work was called the Almagest – a mathematical and astronomical treatise proposing the complex motions of the stars and planetary paths. The book itself was brilliant, covering the problems of daily motion of celestial objects, time spans of daylight, determination of latitude, the points at which the Sun is vertical, the shadows of the sundial’s gnomon at the equinoxes and solstices, and other observations that change with the spectator’s position. It covered retrograde planetary motion, the motion of the Moon, lunar parallax, the motion of the lunar apogee, eclipses, precession of the equinoxes, and motion of the Sun and fixed stars. But very importantly, it gave us our first 48 “official” constellations. And, believe it or not, the 48 constellations Ptolemy described are still used by astronomers today!

However, there was only one small problem with the Almagest and its constellations. The charts that Ptolemy used came from Hipparchus’ time and only included the stars that he could see from his position. It wasn’t long until it was improved greatly by the Islamic culture, whose astronomers described observations of the stars, their positions, magnitudes, brightness and color – even adding drawings for each constellation. Consequently, even today some of our brightest cataloged stars are still referred to by their Arabic names!

Once again, it didn’t take long until the world needed a better system, so along came German astronomer, Johann Bayer, and English astronomer, John Flamsteed, to the rescue. While they did not create more constellations – it was their task to assign Greek letters and numbers (respectively) to the brighter stars in each constellation to make them a standard. It soon became even more confusing as astronomers traveled to the southern hemisphere, adding even more stars and more constellations!

The Modern Constellations

iauFormed in 1919, the International Astronomical Union (IAU) is a collection of professional astronomers whose job it to act as the internationally recognized authority for assigning designations. In 1930 the IAU decided to choose the work of Belgian astronomer, Eugene Joseph Delporte for a worldwide standard. At this point, he fixed the modern boundaries between all of the constellations in the sky, along lines of right ascension and declination for the epoch B1875.0 and the 88 officially recognized constellations went into effect. Because Delport chose such an early epoch for the work of Benjamin Gould, these 80 year old works are no longer perfectly vertical or horizontal thanks to the precession of the equinox, and will one day have to be re-worked. However, a standard was set and constellation names, patterns and borders finally recognized.

What is a Constellation?

A constellation is a recognizable pattern of stars that has official borders and an official designation. There are 88 officially recognized constellations.

Don’t forget that we often recognize star patterns – but these are asterisms and not constellations. For example, the “Big Dipper” is an asterism and part of the larger constellation of Ursa Major – just as the “Great Square of Pegasus” and the “Keystone” of Hercules are not all there is, either. You’ll find many such asterisms as you travel the night… the “Summer Triangle”, “Orion’s Belt”, the “Northern Cross”, the “Southern Cross”, the “Winter Circle”… Just remember…

To be a constellation it has to have official borders and an official designation!

Illustrations: Mythological Constellations created by Stellarium, Zodiac Map courtesy of University of Hong Kong Dept of Physics, Ptolemaic Charts (historic image), IAU Official Constellation map courtesy of IAU/Sky & Telescope Magazine.


Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

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