The Major Constellations:
Andromeda | Aquarius | Aries | Cancer | Capricornus | Cassiopeia | Cepheus | Cetus | Corona Borealis | Cygnus | Draco | Eridanus | Gemini | Hercules | Hydra | Leo | Libra | Lyra | Orion | Perseus | Pisces | Sagittarius | Scorpius | Taurus | Ursa Major | Ursa Minor | Virgo
- Charts and tables
- Constellations & Chandra Images
- Airy Disc under Average Seeing
- Astronomically Correct Twinkle Twinkle
In this section we learn about the constellations, the fundamental divisions of the sky.
Few constellations look like the animal or person they are named after, and you should not be frustrated if you cannot see a princess, bear, or winged horse in the sky. Only in a few cases can the stars be connected to be made to look like a man, a lion, or a swan. Many constellations were named in honor of heroes, beasts, and objects of interest to the people who named them, and not because of any physical resemblance. We do similar things today: the state of Washington looks nothing like the gentleman on the U. S. dollar bill, nor does the bridge in New York City. The exceptions are few, among them Orion, Scorpius, Gemini, and Taurus.
During the Renaissance, when star maps were designed to be beautiful, as well as useful, they were filled with elaborate and colorful drawings that often ignored the background stars. Over time, there was more than one way of drawing the ‘connect-the-dot’ stick figures, which we make to identify the constellations. In any case, there are no “official”, or correct ways to connect the stars, and you are free to invent your own designs if you like.
Some common star patterns are not actual constellations. These asterisms (from aster, the Greek word for star) can be part of one constellation or parts of two or more constellations. These are a few of the major asterisms:
- Big Dipper: The seven brightest stars of Ursa Major.
- Great Square of Pegasus: The three brightest stars of Pegasus and the westernmost bright star of Andromeda.
- Keystone: Four medium-bright stars in Hercules that form a square
- Little Dipper: The seven brightest stars in Ursa Minor.
- Sickle: Stars in Leo in the shape of a harvesting sickle (or a backwards question mark).
- Summer Triangle: The stars Vega in Lyra, Deneb in Cygnus, and Altair in Aquila.
Constellations have both a formal Latin name (Aquarius, for example) and a common English equivalent (Water Carrier). This differs from star names, which are mostly Arabic. Some constellations pre-date the Latin-speaking Romans, but Latin was the language of scholarship until recently, and the Latin names were standardized long ago. New constellations invented in modern times (such as Leo Minor) were given Latin names to conform with ancient custom.
At any given moment you can see half the sky – and approximately half the constellations. As the hours pass and the sky rotates overhead, constellations in the west set and are replaced by others that rise in the east. During the course of a long winter night you can see perhaps two-thirds of the sky, and during the course of a year even more. But – unless you live on the equator – some constellations remain permanently hidden from view. The far southern constellations are a mystery to us in the Northern Hemisphere, and their names – Dorado, Tucana, Centaurus – sound romantic.
Just as the constellations that lie far to the south remain out of sight (assuming you live at a mid-northern latitude), the constellations far to the north remain visible all year. They do not change with the seasons, although they are more easily visible at one time of year than another. Because they are always available, they seem to have less value and we take them more for granted. The Big Dipper is above North America for much of the year and has little novelty – yet Australians never see it.
The most popular and best-known constellations are the 12 that make up the classical zodiac. This is true even though half of the zodiacal constellations contain few, if any, bright stars and are not conspicuous, and several cannot be seen from urban areas. They are well-known because of their astrological associations, but their fame does not correspond to their visibility. It is ironic that everyone has heard of Cancer, although few could find it, while comparatively few people have heard of Auriga, which is a magnificent constellation of bright stars that is filled with star clusters. Just as some people are famous because of where they are (the position they have in our society) rather than who they are, so it is with the constellations. A constellation’s popularity tells you nothing about how interesting it is.
The Big Dipper
The Big Dipper is a good entry point for learning about the positions of certain stars in the night sky. This asterism is circumpolar when viewed from mid-latitudes in the Northern Hemisphere. This means that it never completely sets below the horizon and is always visible in the night sky.
The Big Dipper itself is not a constellation, but is an asterism within the constellation of Ursa Major. Ursa Major is also known as the Great Bear and is the third largest of the 88 constellations. The name originates from the ladle-shaped pattern formed by the seven main stars in the constellation.
This asterism can be found by facing north, as shown in the view in the Main Window. At certain times of the year the Big Dipper will look like a ladle lying upon a table; at other times of the year the ladle will appear to be upside down. The actual shape of the Big Dipper does not change over time, only its apparent orientation.
- In autumn, the dipper appears to be lying flat.
- In spring, the dipper is upside down, “spilling” its contents.
- In summer, it sits upright on its bowl
- In winter, it sits with its handle downwards.
What’s in a Name?
The Big Dipper asterism is primarily seen a ladle in North America. Different cultures, however, see different things when they look at the Big Dipper. In England it is known as the Plough, in Germany as the Great Cart and in western Africa as the Drinking Gourd.
The stars of the Big Dipper act as a handy guide to the positions of other stars and constellations, and to other objects that may be too faint to spot with the naked eye. Using well-known stars in the sky to find fainter objects is known as “star hopping”. You can think of this as an astronomical treasure hunt!
The most useful alignment for observers in the northern hemisphere is produced by the two end stars that form the front of the dipper’s bowl. A line projected from these two stars points directly at Polaris, the North Star. The position on the horizon directly below Polaris is almost exactly north; a very useful direction should you become lost (without your GPS!).
All the other stars in the sky seem to turn counter-clockwise around Polaris. Polaris itself marks the end of the handle of another pattern or asterism, the Little Dipper.
A second star-hopping line from the Big Dipper follows the curved handle of the Big Dipper to two of the northern hemisphere’s brightest spring stars, Arcturus and Spica. This star-hopping technique becomes surprisingly easy with a bit of practice.
Polaris and latitude
Polaris is a very special star for northern hemisphere dwellers and navigators. It is only about 1 degree away from the North Celestial Pole (NCP), the point directly above the north end of the Earth’s spin axis. This easily enables us to orient ourselves on Earth on a clear night.
The North Celestial Pole (NCP) offers an additional navigational benefit, in addition to defining north. An astrolabe is a device that can determine your latitude by measuring the angle between the horizon and Polaris. Astrolabes were regularly used by early explorers and by mariners on sailing ships.
The south star?
People living in the Southern Hemisphere are not quite as fortunate as those in the Northern Hemisphere. There is no bright star the equivalent of Polaris to mark the location of the south celestial pole in their sky.
Measuring the angular distances between stars is easy if you use your hand, held at arm’s length.
With your hand at:
- the width of your little finger is about one degree, enough to cover the Moon and Sun. These objects are each half a degree across.
- the width of the first three fingers side-by-side spans about five degrees.
- a closed fist is about ten degrees.
if you spread out your fingers, the distance from the tip of your first finger to the tip of your little finger is 15 degrees. The distance from your little finger to your thumb covers about 25 degrees of sky.
This hand system, with a bit of practice, can be very useful for measuring angles in the sky.
Calibrating with the Big Dipper
Everyone’s hands are slightly different, so you might want to “calibrate” your own hand measurements using the Big Dipper.
Here are the approximate angular distances from Dubhe to several other prominent Big Dipper stars:
Dubhe to Merak
Dubhe to Megrez
Dubhe to Alioth
Dubhe to Mizar
|Dubhe to Alkaid||
History of the Constellations
Humans have the need, and the skills, to find patterns in nature. Our brains are programmed to invent patterns and to impose order on disorder. This talent helped our remote ancestors find their way around hunting and scavenging sites, and it helps us find our way around the sky.
We also have a need to feel connected to the cosmos. Being completely inaccessible, the sky is an endless source of mystery and wonder. It takes a soulless person indeed to gaze up at the sky on a dark, starry night and not wonder how we fit into it all. People have been doing just that since the beginning of time.
The origins of most constellations are lost in the mists of antiquity, and some of them are prehistoric. We can only unravel as much as we can of the origins and history of the oldest constellations by using the sparse clues available to us. Others were created in more recent times and their history is documented, and the far southern constellations were outlined and named during the Age of Exploration only several centuries ago. As recently as early last century astronomers were proposing new ones, but their number was fixed at 88 early this century and the days of creating new constellations are over.
The core of our familiar western constellations probably originated in prehistoric Sumeria. The Sumerians, who lived in the arid land between the Tigris and Euphrates Rivers in what is now southern Iraq, developed one of the world’s first great civilizations and counted the invention of writing among their achievements. They were a superstitious folk, and they paid great attention to correspondences between events in the sky and events on earth. They saw the seasons as a cyclic battle between the Lion and Bull. In mid-winter 6,000 years ago, Leo the Lion stood high overhead while Taurus the Bull lay “dying” on the western horizon, and the lion was triumphant. The reappearance of the Bull in the morning sky marked the return of spring and the death of winter, and the Bull’s turn to triumph. They charted the stars to try to figure out what was happening to them and to the world around them, and their ideas of interpreting omens were elaborated upon by the Babylonians, who lived in the same area much later. The Babylonians left the first constellation lists on clay tablets, and they invented the idea of the zodiac around the 6th century BC.
The Oldest Constellation?
The Great Bear (the brightest part of which is the Big Dipper) is probably the oldest constellation, and it dates to prehistoric times. Bears were worshipped in “cave man” days in Europe, before they became extinct (in Europe) at the end of the last Ice Age. Bears are still worshipped by nomadic people in Siberia.
People named the celestial Great Bear for its behavior. It does not look like a bear, but it does act like a bear. Earth bears hibernate, and so does the celestial bear. It’s low in the north in winter and returns in spring in a way that reminded people of bears’ seasonal behavior.
What is truly interesting is that widely separated people around the world – from Europe to Asia to North America – saw these stars as a bear. We can only speculate, but it’s likely that the concept of the Great Bear originated during the Ice Age and was carried from Europe to Siberia – or the other way around – and then to North America more than ten thousand years ago. If so, this name is one of the world’s oldest surviving cultural artifacts.
Little Babylonian, and less Sumerian constellation lore remains today, and that which does is imbedded in the classical Greek stories. The Greeks borrowed the concept of the zodiac from the Babylonians and incorporated Babylonian star stories into their own myths. Later, the Romans borrowed the Greek stories and we’ve borrowed those of the Romans, so there is a tradition of borrowing and elaborating that dates back at least 6,000 years.
We know Greek mythology in detail, but less about how they divided the sky. There are only scattered references to stars and star patterns from early Greek times.
The astronomer Ptolemy, who lived in Alexandria around 150 BC, described the 48 “Ptolemaic” or classical constellations, which remained essentially unmodified for 14 centuries. Perhaps 30 of these are Babylonian in pedigree and the rest indigenous Greek. His main source was a long poem based on an earlier and now lost work from about 350 BC by Eudoxus – the Greek astronomer who constructed the first recorded celestial globe and who worked out the idea of celestial coordinates. Ptolemy’s book became known as the Almagest (“the Greatest”) when translated into Arabic. The Arabs gave most of the stars their familiar common names, which are usually Arabic translations of the stars’ positions as described by Ptolemy. Rigel, for example, comes from Arabic for “foot,” which is exactly where the star is within Orion. Following the Dark Ages, the Almagest was translated into Latin, the universal language of the Christian world, and reintroduced into Europe around the year 1,000 after an absence of nearly a millennium. The 48 Ptolemaic star patterns form the core of the constellations of the northern sky.
Many additional constellations were added during the Age of Exploration. When European navigators first ventured into southern waters in the late 1500s and early 1600s, they discovered an uncharted sky in addition to uncharted lands. They divided the southern sky into groups of stars, naming the new constellations after exotic things they found in the new world, like Pavo the peacock and Indus the American Indian. Many of these new constellations achieved legitimacy and permanence by virtue of being included in Bayer’s great Uranometria star atlas of 1603. (Bayer also introduced the idea of using Greek letters to name the brighter stars in this atlas.)
Seven additional constellations appeared in 1690 in a star atlas by the Polish astronomer Johannes Hevelius. He felt that some areas of the sky were too empty and that his atlas would look more attractive if these areas of faint stars only were filled in. His new constellations include Lacerta the Lizard and Vulpecula the Fox. Hevelius is remembered today as the last astronomer to reject using the “newfangled” telescope and to observe by eye instead.
The last burst of constellation-naming is courtesy of the French astronomer Nicolas Louis de Lacaille, who lived in Cape Town, South Africa, from 1750-1752. He created 14 new constellations from the southern stars, naming them after practical objects like an air pump, a chisel, a microscope, and a pendulum clock. This may not seem romantic to us, but apparently these were fascinating objects in their time, and now they too are immortalized in the sky. They contrast so greatly with the mythological beasts and heroes of the classical age that Lacaille has been accused of turning the sky into someone’s attic.
(Images micro.bmp and tele.bmp Caption: “Telescopium and Microscopium, two of the Southern constellations named after technological objects”)
Not all constellations are ancient. Although the last wave of constellation-inventing ended when the southern sky was finally filled in the middle of the 18th century, astronomers felt free to create their own as recently as the late 1800s. Often constellations were created for political purposes – to flatter a patron, for example – but such contrivances were seldom accepted graciously by other astronomers, and most disappeared as quickly as they appeared. They now are minor footnotes in the history of the sky. Examples include Robur Carolinium, or Charles’ Oak, invented by Edmond Halley to honor King Charles II (who once escaped death by hiding in an oak tree); Frederick’s Glory, a sword that honored Prussia’s Frederick the Great; and Telescopium Herschelli or Herschel’s Telescope.
Through the 18th century, there was no universally approved list of constellations and no official constellation boundaries. Mapmakers were free to add new constellations if they wished and to decide where one constellation ended and another began. This situation was intolerable to astronomers, who were creating ever-more detailed star charts, and an early task of the new International Astronomical Union was to settle the constellation boundary question once and for all. A commission was appointed to draw up a list of official constellations and to define their boundaries, and the commission’s results were approved in 1928. The boundaries are a series of “straight lines” in the sky that read like legal property boundaries on earth. Since 1928, no new constellations have been invented.
The Official Cygnus
Cygnus may look like a cross or even a swan, but the actual constellation includes many fainter stars that lie outside the popular stick figure. Since 1928 a constellation has been defined in the same way that a parcel of property is described on earth – by specifying its boundaries as a series of interconnected straight lines. The true definition of Cygnus is everything in the sky that lies within these boundaries (the middle two-thirds of the description is omitted):
- Méridien de 19 h. 15 m. 30 s. de 27° 30′ à 30° 0′
- Paralléle de 30° 0′ de 19 h. 15 m. 30 s. à 19 h. 21 m. 30 s.
- Méridien de 19 h. 21 m. 30 s. de 30° 0′ à 36° 30′ m
- Paralléle de 36° 30′ de 19 h. 21 m. 30 s. à 19h. 24 m.
- Méridien de 19 h. 24 m. de 36° 30′ à 43°
- Paralléle de 28° 0′ de 21 h. 44 m. à 21 h. 25 m.
- Paralléle suite de 28° 0′ de 21 h. 25 m. à 20 h. 55 m.
- Méridien de 20 h. 55 m. de 28° 0′ à 29° 0′
- Paralléle de 29° 0′ de 20 h. 55 m. à 19 h. 40 m.
- Méridien de 19 h. 40 m. de 29° 0′ à 27° 30′
- Paralléle de 27° 30′ de 19 h. 40 m. à 19 h. 15 m. 30 s.
(Reference: Délimitation Scientifique des Constellations, E. Delporte, Cambridge, 1930.)
Constellations of Other Cultures
Our familiar constellations are a product of the history of our western culture. Today, the 88 constellations we know and love are universally recognized. Like our Gregorian calendar, they are “official” around the world. But it was not always so, and each culture invented its own way of dividing the sky. Sadly, the indigenous constellations of most cultures have been lost and only in scattered remote areas are pre-western constellations still remembered.
The ancient Egyptians saw a Crocodile, Hippopotamus, the front leg of a bull (our Big Dipper), and the god Osiris (our Orion). Their Isis is our Sirius – an important goddess whose reappearance was used to predict the annual flood of the Nile. The original Egyptian constellations were replaced by the familiar Greek constellations after Egypt was conquered by Alexander the Great in the third century BC, and in the following centuries their ancient knowledge was almost completely lost. The only clues that remain of ancient Egyptian sky lore comes from enigmatic tomb paintings.
The Chinese divided the ecliptic into 28 lunar mansions, somewhat analogous to our zodiac (which is solar), and the stars into almost 300 groupings that are smaller than our constellations and that we would call asterisms. Some patterns that look so obvious to us that we have a hard time seeing them any other way, like the W of Cassiopeia, are subdivided by the Chinese (in the case of Cassiopeia, into three). They have four seasonal “super-constellations,” each made of several asterisms with a similar theme: the White Tiger of autumn, the Black Tortoise of winter, the Blue Dragon of spring, and the Red Bird of summer (illustrations of 4 Chinese super constellations?). The Chinese paid less attention to star brightnesses than we do when dividing the sky and they incorporated fainter stars. We are amazed that Lynx is an official constellation, so faint are its stars, but the Chinese regularly included such faint stars in their asterisms. Among the Chinese constellations are the Dogs, the Awakening Serpent, the Wagging Tongue, the Tortoise, the Army of Yu-Lin, the Weaver Maid, and the Cow Herder.
Peruvian Dark Constellations
The Inca of pre-conquest Peru recognized what we may call “dark constellations.” At their southern latitude, the center of the Milky Way passes straight overhead and is spectacular. They recognized patterns of bright stars like we do, but they also named the dark dust clouds of the Milky Way. They thought of the dark clouds as earth carried to the sky by the celestial river. Among the dark clouds were Ya-cana the Llama and her baby Oon-yalla-macha, the fox A’-toq, the bird Yutu, and Hanp-á-tu the Toad. The Indians watched for changes in the visibility of the dark clouds, caused by upper-atmospheric moisture, that would tell them whether the coming year would be wet or dry.
The Changing Constellations
Remember that constellations are areas of the sky, and they have infinite depth, so the stars in a constellation do not necessarily have anything to do with each other, although they appear to lie in approximately the same direction as seen from earth. Two stars that appear to be very close to each other may actually be separated by enormous distances, one far beyond the other, while stars on opposite sides of the sky may be relatively close to each other with us in the middle. You cannot tell just by looking. This third dimension of depth means that the appearance of the constellations depends on your vantage point.
Distances within our solar system are so small that the constellations look exactly the same from Mars, Venus, and Pluto. But if we move far beyond our solar system, the story changes. If our earth orbited a distant star rather than our sun, the stars would be distributed in completely different patterns and our familiar constellations would not exist. If aliens live on other planets in orbit around other stars, they have their own constellations.
In addition to all the other adjustments that come with moving into a new home, humans of the future who colonize planets around other stars will have the task of dividing their sky into their own constellations. It will be a lot of fun.
One of the nearest stars to Earth which we may visit is Barnard’s Star, about six light years distant. Open the file “Barnard” to see that the new immigrants would see a bright 0th magnitude star near the belt of Orion which is not in our night sky – because it is our own sun!
Even when viewed from earth, the constellations are not forever. Each star in the sky is actually in motion. Although these speeds are quite high (most stars at moving at several hundred kilometres per second!), the vast distance to the stars mean that this motion is imperceptible to us. Each star’s speed and direction is different from its neighbors, causing each star to move relative to its neighbors as seen from earth, over very long periods of time. This very slow motion of a star across the celestial sphere is called its proper motion (“proper” meaning “belonging to,” rather than “correct”). Each star’s info in the RASC’s Observer’s Handbook shows that star’s proper motion in terms of the amount by which a star’s RA and Dec will change each year. (Remember that a star’s RA and Dec will also change due to precession, but precession changes the positions of all stars equally.)
Our familiar constellations look much the same now as they did at the end of the last Ice Age, but in the distant future they will become distorted by their stars’ motions and eventually they will become unrecognizable. The constellations of 1 million AD will bear no resemblance whatsoever to the star patterns we know and love today. Eventually people will have to invent new constellations. It would be interesting to know how long people will retain the classical constellations as they become increasingly distorted before revising the scheme and starting over.
The Big Dipper 100,000 Years from Now
The Big Dipper really does look like a dipper, but it won’t always. The middle five stars are traveling together through space on nearly parallel paths as they orbit around the center of the Milky Way, and they will retain their relative spacing far into the future. The Dipper’s two end stars, however, are traveling in the opposite direction. Eventually the Big Dipper will become stretched into what people may one day call the Big Lounge Chair.