The study of celestial objects is an ancient one. Knowledge of the sun, moon, and stars, and their associated mythology, was passed from generation to generation but few conclusive records of prehistoric observations survive.
Constellations were part of the historical record in Mesopotamian culture around 4000 B.C. In the 8th century B.C. Homer mentioned a few now familiar constellations in his epic poem, the Odyssey. Four hundred years later Eudoxus of Cnidus wrote about 43 constellations (or 45 or 48 depending on one’s interpretation) which survive today. Eudoxus’ original work was lost but his ideas were kept alive by Aratus in a poem called Phaenomena (circa 270 B.C.).
The Classical Constellations
Many of our present day constellations can be found in a book called the Almagest, written circa 150 A.D. by Ptolemy, an Alexandrian astronomer. Ptolemy made his own celestial observations from about 120-150 A.D. but also used historical data. He stated that the oldest astronomical record he had access to was from Babylonia in the 8th century B.C. While some of the star data in the Almagest was his own Ptolemy certainly got part, and perhaps most, from Hipparchus, a 2nd century B.C. Greek astronomer.
The 48 constellations listed in Ptolemy’s Almagest are:
21 Northern Constellations
Andromeda, Aquila, Auriga, Boötes, Cassiopeia, Cepheus, Corona Borealis, Cygnus, Delphinus, Draco, Equuleus, Hercules, Lyra, Ophiuchus, Pegasus, Perseus, Sagitta, Serpens, Triangulum, Ursa Major, Ursa Minor
12 Zodiacal Constellations
Aries, Aquarius, Cancer, Capricornus, Gemini, Leo, Libra, Pisces, Sagittarius, Scorpius, Taurus, Virgo
15 Southern Constellations
Ara, Argo Navis, Canis Major, Canis Minor, Centaurus, Cetus, Corona Australis, Corvus, Crater, Eridanus, Hydra, Lepus, Lupus, Orion, Pisces Austrinus
All of the Ptolemaic constellations are still in use today. The only one you may not be familiar with is Argo Navis (the Ship Argo). In our modern constellations the Ship has been broken up into smaller parts, Carina (the Keel), Puppis (the Stern), and Vela (the Sail).
Al-Sufi, a Persian astronomer (mid-10th century A.D.), reobserved the Ptolemaic stars in the Almagest and corrected their magnitudes.
A famous Arabic astronomy text, the Alfonsine Tables, was completed by scholars in Moorish Spain in 1252. It included an updated version of the star coordinates found in Ptolemy’s then 1100 year old Greek text. The Alfonsine Tables was translated into many other languages and spread throughout the rest of Europe.
Two hundred years later the Almagest was again updated, this time by the great Tatar astronomer Ulugh Beg. He worked from the city of Samarkand (in what is now Uzbekistan) observing and correcting stellar coordinates to the contemporary epoch. Ulugh Beg’s 1437 Zig Tables was not to be superseded until Tycho Brahe’s star catalog of 1602.
The Modern Constellations
For well over a thousand years the Almagest was taken as the last word on the stars. In spite of the fact that stellar positions were updated from time to time no astronomer had made any lasting addition to Ptolemy’s constellation list. That finally changed in 1536 when German globe maker Caspar Vopel added two new northern constellations, Coma Berenices and Antinous, to the classical forty-eight. The stars in both figures had been cataloged earlier by Ptolemy but had not been set up as distinct constellations. Vopel’s Coma Berenices survives today but Antinous does not.
The forty-eight classical constellations did not cover the entire sky. From the latitude of ancient Greek observatories the extreme southern stars never rose above the horizon. That part of sky, the southern celestial void, was left blank on celestial maps until the 16th century.
On his 1589 map the Dutch astronomer Plancius began to fill the southern celestial void by creating two constellations, Crux and Triangulus Antarcticus. Plancius went on to invent new constellations at a rapid pace. In 1592 he made Columba from Ptolemaic stars and Polophylax from more recent observations. Only Columba was kept by astronomers. Six years later Plancius added more new constellations to the southern sky using stars plotted by Keyser, a Dutch navigator. Plancius did not stop there. In 1612 he used recently charted stars to add eight more constellations, primarily to the northern sky. Only two of those constellations are still recognized.
In the later 17th century the German/Polish astronomer Hevelius added more constellations to the sky. 64 years later LaCaille, a French astronomer, added additional constellations. Finally, LaCaille and de Vaugondy broke up Argo Navis into three parts and these became the last of the officially recognized constellations.
Many other astronomers from the 17th, 18th, and 19th centuries invented constellations but most were quickly forgotten. In 1801 the German astronomer Johann Bode published a set of celestial maps which included 99 constellations and a handful of smaller asterisms. Bode’s work was the last great star atlas to illustrate so many constellations. Among his new figures was a static electricity machine, a hot air balloon, and a cat. These constellations now exist only in history books.
In 1919 a group of astronomers formed the International Astronomical Union or IAU. One of the first items they tackled was the celestial disarray. The IAU made a list of 88 ‘official’ constellations in 1922 and approved the boundary of each six years later.
The 88 official constellations chosen by the IAU were all of European origin because those constellations were already well known and used by many professional astronomers.
The official constellations are no more and no less important than the constellations of any other time, country, or culture. What they are is the common language for astronomers around the world. Think of the world’s spoken languages – there is no best or correct language and each conveys some meanings better than others. Still, English is becoming the lingua franca and like English the official constellations are a useful tool for international communication.
Learn the heritage of your own culture’s constellations and get to know the star lore of other cultures. They too fill the sky with wonder!
Finding the Modern Constellations
The modern constellations can be a challenge to observe. The classical constellations of ancient Greece include almost all of the brighter stars in the sky as seen from the Mediterranean region. The stars near the south celestial pole remained below the local horizon and so astronomers could not catalog them. [It is interesting to note that because of precession – a wobble in the earth’s rotational axis – some stars in the ancient celestial void can now be easily seen from Greece. Contrariwise, some stars that were visible to ancient Greek astronomers can not be seen from those latitudes today.] The southern celestial void was first partially filled by Plancius and Keyser and they used most of the brighter stars. More than 150 years later LaCaille mapped near the same area of southern sky but his constellations are mostly very dim because the brighter stars had already been taken. Hevelius’ northern constellations are also quite dim because he too had to use leftover stars (the classical constellations had already used the brighter stars).
Find a dark site to observe the modern constellations. I can not emphasize that enough! It will be much less frustrating if you are not hampered by light pollution. Make sure you have a good map of the stars and practice star-hopping to the dimmer stars. Once you get good at star-hopping with the naked-eye then practice with binoculars. A cheap pair of 7×35 binoculars work fine and are very helpful in locating the magnitude 4, 5, and 6 stars of the modern constellations.
Many modern constellations will be near the south or north horizon and some will never rise very high even at their greatest altitude. The light of a star near the horizon travels through more atmosphere than a star overhead. The more atmosphere a star’s light has to traverse the dimmer it becomes (it also twinkles more). This dimming effect is called ‘extinction’. A star can easily lose 3 magnitudes of brightness when it is just above the horizon as compared to when it is overhead. For example, a star listed as 5th magnitude (zenith being assumed) may have an extinction magnitude (also called apparent magnitude) of only 8 when it skirts the horizon. Binoculars are a must when a constellation’s stars are near the horizon and “lost in the murk”.
If you get to know the modern constellations well you can often pick them out by looking directly at their stars but for the most part it is easier to use brighter stars as guides. If you wish to find all the constellations it would be good if you first learned the bright classical constellations – they will be your star guides to the dim modern constellations.