Constellation

• 1. Introduction

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  A constellation is a patch of the celestial sphere where visible stars form a perceived outline, usually an animal, a mythological figure or an everyday object. Consider Orion and Scorpius. In myth, the scorpion assassinates the hunter, and the two constellations appear at opposite times of year. Some of these old stories seem to echo the sky itself. The word comes from the Late Latin cōnstellātiō, meaning a set of stars, and it entered Middle English in the 14th century. The Ancient Greek word was astron. Yet the stars in a single pattern usually lie at very different distances from Earth. They only look like neighbors. So who first drew these shapes, and how did many cultures with their own skies end up sharing one official map? And why does a pattern that feels permanent turn out to be temporary?

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• 2. Mesopotamian Roots

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  Some 17,000 years ago, painters in the Lascaux caves of southern France may have recorded the night sky. Suggested readings include Taurus, Orion's Belt and the Pleiades. Most scientists do not accept this view.

  Inscribed stones and clay tablets from Mesopotamia, in modern Iraq, give the earliest generally accepted evidence. They date to 3000 BC. Most Mesopotamian constellations seem to have been created in a fairly short interval, from about 1300 to 1000 BC.

  The oldest Babylonian catalogues of stars go back to the start of the Middle Bronze Age. The best known are the Three Stars Each texts and the MUL.APIN. The MUL.APIN is an expanded and revised version from around 1000 BC, built on more accurate observation. Its many Sumerian names suggest older traditions from the Early Bronze Age that left no record of their own.

  The classical zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. Its astrological divisions became prominent around 400 BC in Babylonian or Chaldean astronomy. The deeper origins of the zodiac remain historically uncertain.

  The biblical Book of Job also names star groups, using the Hebrew words 'Ayish, chesil and chimah. These mean a bier, a fool and a heap. The King James translators rendered them as Arcturus, Orion and Pleiades, but 'Ayish actually corresponds to Ursa Major. A fourth term, Mazzaroth, appears only once, in Job 38:32. It is translated as a garland of crowns and might refer to the zodiac.

  The biblical scholar E. W. Bullinger went further. He read the Lion, Bull, Man and Eagle of Ezekiel and Revelation as Leo, Taurus, Aquarius and Scorpio.

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• 3. Greek and Egyptian Skies

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  The poet Hesiod, in his Works and Days, mentions the heavenly bodies, and that fragment is nearly all that survives about the earliest Greek constellations. Greek astronomy essentially took over the older Babylonian system. Eudoxus of Cnidus introduced it to Greece in the 4th century BC.

  The original work of Eudoxus is lost. It survives only because Aratus turned it into verse in the 3rd century BC. Twenty of the constellations that Ptolemy later listed come from the Ancient Near East. Another ten use the same stars under different names.

  The fullest accounts of the myths behind the constellations come from two writers known only as pseudo-Eratosthenes and pseudo-Hyginus. The Roman poet Ovid gave the subject its most famous treatment in his Metamorphoses.

  Ptolemy, a Greco-Roman astronomer working in Alexandria, Egypt, wrote the Almagest in the 2nd century. It lists 48 traditional constellations: the zodiac plus 36 more. Today the count of non-zodiac figures stands at 38, because one of them has since been divided into three. The Almagest underpinned Western astronomy from Late Antiquity into the Early Modern period. It also remained the standard definition of constellations in medieval Europe and in Islamic astronomy.

  In the Ptolemaic Kingdom, native Egyptian tradition already used human-shaped figures for planets, stars and constellations. Some were merged with Greek and Babylonian systems. The result is the Zodiac of Dendera, the oldest known depiction of the zodiac showing all the familiar constellations. It also carries original Egyptian constellations, decans and planets. It is unclear exactly when this happened, but most of it was placed during the Roman period, between the 2nd and 4th centuries AD.

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• 4. The Chinese Sky

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  Oracle bones from Anyang, dating to the middle Shang dynasty, carry star names that were later sorted into the Twenty-Eight Mansions. Chinese constellations themselves go back to the 5th century BCE. The system grew up independently of the Greco-Roman one. Parallels with ancient Babylonian astronomy hint that the two may have influenced each other earlier on.

  During the Han period, three schools of astronomy were traced back to astronomers of the earlier Warring States era. In the 3rd century, during the Three Kingdoms period, the astronomer Chen Zhuo merged their constellations into one system. His own work is lost. We know his system through Tang dynasty records, especially the Nine Seizers Canon by Gautama Siddha, an Indo-Chinese resident of Chang'an. The oldest surviving Chinese star chart dates to that same period and was preserved among the Dunhuang manuscripts.

  The Suzhou Astronomical Chart, a famous map from the Song period, carves the stars onto a stone plate showing the Chinese sky. It was made accurately from observation. It also records the supernova of 1054 in Taurus. Under the Yuan and Ming dynasties, Chinese astronomy drew on other eastern and western astronomers, and maps grew more scientific and more reliable.

  In the late Ming dynasty, European influence led to charts with more stars while the traditional constellations stayed. Xu Guangqi and the German Jesuit Johann Adam Schall von Bell made further improvements, recorded in the Chongzhen Lishu of 1628. Traditional Chinese star maps took in 23 new constellations with 125 stars of the southern hemisphere, drawn from Western charts. With that step, the Chinese sky joined world astronomy.

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• 5. Charting the South

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  The Farnese Atlas, a statue that carries the oldest representation of a star chart, shows how the northern sky was drawn. It may be based on the star catalogue of the Greek astronomer Hipparchus. Most northern constellations date to antiquity, with names from Classical Greek legends. Southern constellations are more modern inventions, sometimes made as substitutes for ancient ones, as with Argo Navis.

  Below about -65° declination, the sky was only partly catalogued by the Babylonians, Egyptians, Greeks, Chinese and Persians of the north. Classical writers knew that northern and southern patterns differed. They describe an expedition around Africa commissioned by the Egyptian Pharaoh Necho II around 600 BC, and the voyages of Hanno the Navigator around 500 BC.

  The southern constellations mattered most from the 14th to the 16th centuries, when sailors steered by the stars. The Italian explorers Andrea Corsali, Antonio Pigafetta and Amerigo Vespucci all recorded new ones. Different observers proposed different groupings and names. Some reflected national traditions, and some were designed to flatter sponsors.

  Many of the southern figures among today's 88 first appeared on celestial globes introduced in the late 16th century by Petrus Plancius. He relied mainly on the observations of the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman.

  Nicolas Louis de Lacaille, a French astronomer, studied the southern stars from the Cape of Good Hope in 1751 and 1752. He was said to have observed more than 10,000 stars. He created fourteen new constellations and divided Argo Navis into three. They appeared in his catalogue of 1756. Some southern names proved too long to survive. Musca Australis became simply Musca.

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• 6. Shapes in the Dark

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  The Great Rift is a series of dark patches in the Milky Way, and it is most visible in the southern sky. Some cultures saw figures in these patches rather than in the bright stars around them.

  The Inca identified various dark areas, or dark nebulae, as animals. They linked the appearance of those animals to the seasonal rains.

  In Australian Aboriginal astronomy, the best-known dark cloud constellation is the emu in the sky. Its head is formed by the Coalsack, a dark nebula, rather than by any star.

  Other dark regions that have been catalogued as dark cloud constellations include the Cygnus Rift, the Serpens-Aquila Rift, the Dark Horse and the Rho Ophiuchi cloud complex.

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• 7. The Eighty-Eight

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  In 1603, Johann Bayer's star atlas Uranometria assigned each star to a single constellation. He labeled the stars within each one with a series of Greek and Latin letters, still known as Bayer designations. Later atlases led step by step to today's modern constellations.

  Several proposals did not survive. The French astronomers Pierre Lemonnier and Joseph Lalande suggested constellations that were once popular and have since been dropped. The northern figure Quadrans Muralis lasted into the 19th century, when its name was attached to the Quadrantid meteor shower. It is now divided between Boötes and Draco.

  Before precise boundaries were drawn, starting in the 19th century, constellations were ill-defined regions. Observers grouped stars differently, and it was often unclear which constellation a celestial object belonged to.

  In 1922, the International Astronomical Union accepted a list of 88 constellations. Henry Norris Russell produced the list with three-letter abbreviations, but it had no clear borders. It rests roughly on the traditional Greek figures of Ptolemy's Almagest and Aratus' Phenomena, with early modern additions to cover the southern sky unknown to Ptolemy.

  In 1928, the union formally adopted boundaries developed by Eugene Delporte, and the list was published in 1930. The boundaries follow vertical and horizontal lines of right ascension and declination, and together cover the entire celestial sphere. Delporte built on a suggestion by Benjamin A. Gould, whose Uranometria Argentina used the epoch B1875.0. Of the 88 constellations, 36 lie mostly in the northern sky and 52 in the southern. Because every point on the sphere lies in exactly one of them, naming systems can use them. A Flamsteed designation, for example, is a number plus the genitive form of the constellation's name.

  Gould's old epoch has a cost. Because of the precession of the equinoxes, the borders on a modern J2000 map are already somewhat skewed and no longer perfectly vertical or horizontal. The distortion will keep growing over the coming centuries.

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• 8. Reading the Night Sky

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  The Big Dipper in Ursa Major and the teapot in Sagittarius are asterisms. These are star patterns that are not constellations under the formal definition, though observers use them to find their way. An asterism may be a few stars within one constellation, or it may share stars with several.

  Polaris, the North Star, is the approximate center of the northern celestial hemisphere. It belongs to Ursa Minor and marks the end of the Little Dipper's handle. From about 35° north in January, Ursa Major appears to the northeast and Cassiopeia to the northwest. Orion and Taurus stand high to the south. In July, from the same latitude, Scorpius sits to the south, Cygnus with its Northern Cross rises in the east, and Hercules and Corona Borealis ride high.

  The far south has no bright pole star. Sigma Octantis comes closest to being one, but it is faint. Observers instead triangulate the pole using the constellation Crux and the stars Alpha and Beta Centauri, which lie about 30° counterclockwise from Crux.

  The zodiac runs through all of this. Six zodiac constellations lie in the northern sky: Pisces, Aries, Taurus, Gemini, Cancer and Leo. Six lie in the southern: Virgo, Libra, Scorpius, Sagittarius, Capricornus and Aquarius. Earth's 23.5° axial tilt means the ecliptic appears overhead from latitudes 23.5° north to 23.5° south. The Milky Way's plane is inclined 60° from the ecliptic. The Galactic Center lies near Scorpius and Sagittarius.

  None of these pictures is fixed. Each star has its own independent motion, so every constellation changes slowly. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can already reconstruct past and future outlines by measuring the proper motions of individual stars with astrometry and their radial velocities with spectroscopy.

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