The History of the Zodiac
By Mark Hurn
Part I : Circles in Space : Astronomical Basics
In my talk "The Story of Star-Names" I often get questions about the constellations of the zodiac and how they came to be invented. This talk is a direct result of that interest. I hope to explain, what is the zodiac, how it may have developed over time and how it is still of interest to astronomers.
Nowadays, at any mention of the zodiac most people will think of astrology. This talk is an attempt to reclaim the zodiac for astronomy.
Before I go on I must just make some things clear, when I talk of circles in space or of constellations being ‘formed’. I mean ‘formed’ in the human head as models to understand the universe, not the physical origin of the solar system or stars.
Another introductory concept is the ‘celestial sphere’. Ancient Greek astronomers considered the Earth to be at the centre of the universe with the planets, sun and moon going around it carried by spheres, the final outside sphere carrying the stars.
The planets are thought to have evolved from a disc of material surrounding the sun. Possibly as a result of this the planets all lie very close to the plain of the solar system. If we look at a table showing the orbital inclination of the planets, we note that only Pluto has a sizeable inclination.
The result of this is that from the Earth, the sun, the moon and all the naked-eye visible planets can be seen within a narrow belt of sky. This is why you will not see planets in far northern constellations such as the Great Bear (or indeed far southern constellations too).
The apparent movements of the Sun
It is important to note that from the position of an observer on Earth, the sun will appear to have two movements: daily movement and yearly movement.
The daily movement is easy to explain. The sun appears to rise in our East in the morning, rises to a high point in the sky at mid-day, then set in the West. This movement is a result of the Earth rotating.
The yearly movement of the sun is harder to explain. Because the Earth goes around the sun in a period of one year, from the point of view at the Earth, the sun appears to move against the background of stars, completing one circuit in a year. Of course, it is only possible to see where the sun is amongst the stars during a total eclipse, but it is fairly easy to work out where it is from observing the stars at night. So for example, in the middle of the night it is easy to work out that the sun is ‘in’ a directly opposite constellation to that visible.
As the stars are very much further away from us than the sun, this is only a ‘line of sight’ effect.
It is possible to plot the apparent path of the sun through the stars and this path makes up a circle called the ‘ecliptic’. It is called the ecliptic because it is only on this circle that ‘eclipses’ can occur.
For early civilisations the stars on or near the ecliptic where of particular use. At night they could use them to tell the time; ‘how long before sunrise?’. They could also tell the time of year; ‘how long before sowing time?’. So, these stars had a practical use, and it should be no surprise that they were associated into constellations, for ease of identification.
The connection of the ecliptic circle with time led to its division into twelve constellations associated with twelve months of the year. The whole circle being divided into 360 degrees, one degree corresponding roughly to a day of the year.
The band of sky 7 or 8 degrees above and bellow the ecliptic is called the zodiac. Within this band the sun, moon and all major planets visible from Earth (except Pluto) can be seen.
The Celestial Equator
As we mentioned earlier, the Earth rotates around its axis once a day. The axis of rotation, however, is not 90 degrees to the ecliptic, but is tilted by 23½ degrees. This tilt gives us our seasons on Earth.
The tilt of 23½ degrees means that a second circle in the sky is used, this is the celestial equator. The celestial equator is a projection of the Earths equator onto the sky. Because of the tilt the two circles of the ecliptic and the celestial equator cross at two points. This can be seen in diagrams of the celestial sphere.
Equinoxes and Solstices
Where the circles cross we have the equinoxes, at these points night and day on Earth are of equal length. The spring or vernal equinox is where the sun appears to cross above the celestial equator, the autumnal equinox is where the sun appears to cross down bellow the celestial equator. At the highest point of the ecliptic above the celestial equator we have the summer solstice and our longest day. At the lowest point of the ecliptic we have the winter solstice and the shortest day.
The spring or vernal equinox was used as a starting point for the zodiac. It was the beginning of the first sign of the zodiac, Aries (the Ram) and this point is referred to as the ‘first point of Aries’.
Old star atlases often show elaborate celestial spheres showing various circles in the sky including the ecliptic and celestial equator. On some, the tropics of Cancer and Capricorn are shown along with the observers horizon.
Unfortunately there is also a third movement, which has to be taken into account. We have seen how the Earth turns on its axis once a day, but it also has a slight wobble as it turns, so that the axis itself rotates once in 25,800 years. Although this may sound like a very long time, human civilisation has been around for long enough for this movement (called precession) to be noticed.
As Sir Patrick Moore has observed, when the Egyptian pyramids were being built (around 2500 BC) the north pole star was Thuban (in Draco – the Dragon) rather than our familiar Polaris.
The second part of this talk will show how precession has affected the zodiac.
Part II : the Historical Development of a Zodiac
For the earliest recorded mention of the zodiac we must go to ancient Messopotamia (roughly the area of modern Iraq). The Messopotamians (particularly the people of Babylon) recorded astronomical events on clay tablets, many of which have been unearthed by archaeologists. These tablets first record the use of the zodiac in the 5th century BC.
Other evidence for a Mesopotamian origin for the zodiac exists in the types of animals chosen for the constellations (e.g. lions and scorpions).
From Mesopotamia the zodiac spread into Egypt and Greece.
The earliest depiction of the zodiac we have is from Egypt. The ‘Dendera Zodiac’ is a carving on sandstone from the temple of Hathor in Dendera. It is now held in the Louvre Museum, Paris and is dated to the 1st century BC. It shows the twelve constellations of the zodiac we are familiar with as well as the Egyptian system of ‘decans’ – 36 constellations each representing ten days of the year.
Another Egyptian depiction of the zodiac can be found on the inside of a coffin lid found in West Thebes. It depicts the sky goddess, Nut (often identified with the Milky Way) surrounded by pictures of the signs of the zodiac. This is now held in the British Museum.
The famous Greek astronomer Hipparchus (who is credited with discovering precession) was probably instructed by Babylonian astronomers and helped perpetuate the usage of the zodiac in the Greek world.
The most influential astronomer of the ancient world was Ptolemy who described the zodiac in his great book on astronomy the Almagest. Since this one book dominated astronomy up to the 16th century, the zodiac was preserved through to modern times.
But can we take the history of the zodiac back further in history? That is to before the 5th Century BC in Babylonia?
A Russian professor of the history of science, Alexander Gurshtein, has proposed a theory that the zodiac was not originally twelve signs but just four. We have seen how there are four important points on the ecliptic, the vernal equinox, the summer solstice, the autumn equinox and the winter solstice. Gurshtein suggests that around 6-5000 BC these points were aligned with four constellations of the zodiac.
Gemini vernal equinox
Virgo summer solstice
Sagittarius autumn equinox
Pisces winter solstice
The effect of the precession of the equinoxes over two thousand years then shifted these points out of these constellations, and caused the early astronomers to make up new constellations, so that by 4-3000 BC the position was:
Taurus vernal equinox
Leo summer solstice
Scorpius autumn equinox
Aquarius winter solstice
Another 2000 years on and we move into the arrangement we are familiar with:
Aries vernal equinox
Cancer summer solstice
Libra autumn equinox
Capricornus winter solstice
It is a neat theory and evidence supporting it is offered. Gurshtein looks at the size of constellations (in square degrees) and suggests the older a constellation is, the bigger it is. He demonstrates that the earliest constellations in his theory (Gemini, Virgo, Sagittarius and Pisces) are on average larger than the next four (Taurus, Leo, Scorpius and Aquarius) and the last four (Aries, Cancer, Libra and Capricornus) smaller still. He also uses evidence from Mesopotamian mythology; note how the constellations marking the winter solstice are watery: Pisces (the fishes), Aquarius (the water carrier) and Capricornus (the sea-goat). This, he suggests could indicate their immersion in the subterranean ocean of Mesopotamian mythology. There is also the case of Sagittarius and Scorpius, ‘stinging’ or ‘arrowing’ the sun from the sky at the autumn equinox.
Finally, there is evidence that Libra is a later constellation, as some early sources label it as the claws of Scorpius.
At one time I thought a Mesopotamian ‘astrolabe tablet’ held in the British Museum might provide evidence for Gurshteins ideas. It is meant to depict the zodiac, but appears to contain eight divisions not twelve, although dated to the 7th century BC, could this represent the earlier system of 4-3000 BC with just eight zodiac constellations? Unfortunately, it is incomplete and only two of the stars identified are in the zodiac, so its purpose remains a mystery.
Whether we accept Gurshteins ideas or not, the continuing effect of precession has led to a major change in how we regard the zodiac.
Part III : Signs and Constellations
I mentioned in the first part of this talk how Aries was the first in the order of the zodiac constellations and that the ‘First Point of Aries’ marked the vernal equinox. However, because of precession, the vernal equinox has shifted into the constellation of Pisces, however, this point is still called the ‘First Point of Aries’.
Precession has led us to have to make a distinction between ‘zodiacal constellations’ (these are the actual stars we see in the sky) and the ‘zodiacal signs’ which are now merely 30 degree segments of the ecliptic.
Those who claim Ophiuchus as the ‘13th sign of the zodiac’ are failing to make this distinction. Ophiuchus (the Serpent Holder) has a few stars which cross south of the ecliptic between Sagittarius and Scorpius, but cannot be considered a ‘sign’ of the zodiac for that reason.
The signs of the zodiac are now really only used by astrologers and most people are aware of the sign the sun was in when they were born, even if they cannot see any truth to astrology itself.
The word zodiac is meant to derive from the Greek meaning ‘animal signs’ (think of ‘zoology’ – the study of animals), although not all the signs are animals. Each sign traditionally has a ‘sigil’ (astronomical short hand) associated with it.
The zodiac is illustrated in my talk by a slide showing a map of the heavens drawn by Albrecht Durer (a 16th century German artist).
Mythology of the Zodiac
Each constellation of the zodiac has at least two or three myths associated with it (mainly from Greek mythology).
ARIES (The Ram)
It’s fleece was the Golden Fleece sought by Jason and the Argonauts. It was originally a magic flying animal made by the god Hermes.
TAURUS (The Bull)
One myth has it that the great god Zeus disguised himself as a beautiful white bull to seduce Europa, the daughter of King Agenor of Phoenicia.
GEMINI (The Twins)
The twins, Castor and Pollux were inseparable friends, one mortal, one immortal, when Castor (the mortal twin) was killed, his brother Pollux was so grief-stricken that he begged Zeus to let him share his immortality with his brother. Zeus allowed this by placing them both together in the sky.
CANCER (The Crab)
This crab was a minor character in the story of the hero Hercules. The crab was sent by the goddess Hera to attack Hercules as he fought with the Hydra (The Water Snake). Crushed by Hercules, the crab was rewarded by Hera with a place in the sky.
LEO (The Lion)
The Lion of Nemea, killed by Hercules as the first of his famous twelve labours.
VIRGO (The Virgin)
Myths link this constellation with the goddesses of justice, Astrea and Dike and many other female characters including Persephone. She is normally depicted as holding an ear of wheat (marking the star Spica).
LIBRA (The Scales)
According to some sources this constellation was part of Scorpius forming the claws of the scorpion, or part of Virgo being the scales of justice held by the goddess.
SCORPIUS (The Scorpion)
The Scorpion is said to have killed Orion (the Hunter). As the constellation of the scorpion rises in the east, Orion sets in the west.
SAGITTARIUS (The Archer)
This constellation is depicted as a centaur shooting an arrow, not to be confused with the other half-man half-horse Centaurus. Sagittarius may have its origins in the Mesopotamian archer god – Nergal.
CAPRICORNUS (The Sea Goat)
The goat-fish, sea-goat or horned goat has various myths and is sometimes associated with the Greek god Pan.
AQUARIUS (The Water Carrier)
This constellation is shown as a man pouring water from a jar, various myths link him with Hapi (god of the Nile) or Ganymede the cup-bearer to the Greek gods.
PISCES (The Fishes)
Possibly inspired by the story of Venus and Cupid escaping from a giant called Typhon by jumping into the River Euphrates and disguising themselves as fishes.
Before I conclude with a section on observing the zodiac. I think for completeness I should mention a similar, and possibly much older, system to the zodiac. This is the system of lunar mansions. These are groupings of stars marking the daily motion of the moon in its orbit. This system was widely used in the ancient astronomy of Arabia, China and India. In accordance with the length of the lunar month (from new moon to new moon: 29 days 12¾ hours) and the sidereal revolution of the moon (from one star and then back: 27 days 7¾ hours) there were normally 27 or 28 lunar mansions. This system had an obvious use in lunar-based calendar keeping.
Part IV : Observing the Zodiac
You can see a few constellations of the zodiac on any night of the year. In the winter, Taurus and Gemini are well placed for observation. In the spring, Cancer, Leo and Virgo can be seen (take a long curve from the tail of Ursa Major through Arcturus will take you to the bright star Spica in Virgo. In summer, Libra and Sagittarius can be seen. The autumn, is probably the best time for Aries, Aquarius, Capricornus and Pisces, although these constellations are never that prominent for British astronomers.
It is worth learning the constellations of the zodiac as this can help you identify and observe the planets. If a familiar constellation of the zodiac, somehow looks odd, this may be because you are seeing one or more planets within it.
There are six first magnitude stars in the zodiac, these are Aldebaran (Taurus), Castor and Pollux (Gemini), Regulus (right on the ecliptic in Leo), Spica (Virgo) and Antares (in Scorpius).
Cancer does not have any bright stars, but does have the famous star cluster, M44 variously called Praesepe or the ‘Beehive Cluster’ is very impressive through binoculars.
The Zodiacal Light
The zodiacal light is a special phenomena connected with the zodiac. It is best seen after sunset in the spring, or before sunrise in the autumn. At these times the ecliptic is at it’s steepest angle to the horizon. You need dark skies, but can make out a pale cone of light along the line of the ecliptic. It is caused by light scattered from tiny meteoric particles spread around the plain of the solar system. In very good conditions it can be seen extended right across the sky as the zodiacal band.
There are some important meteor showers connected with the zodiacal constellations.
In May there are the eta-Aquarids in Aquarius (maximum 5-6 May, 40 ZHR). In July there are the delta-Aquarids also in Aquarius (maximum 28-29 July, 20 ZHR). Then in early November there are the Taurids (maximum 3 November, 12 ZHR). In mid November we have the Leonids, the remains of the comet Tempel-Tuttle. These give spectacular displays roughly every 33 years, the last being in 1998 (the previous in 1966). But on a normal year the maximum around 17-18 of November might only have a ZHR of 10. Finally there are the Geminids in December (maximum 13-14 December, 60 ZHR).
I hope this talk goes some way to interesting you in the zodiac. There are certainly many connections linking the zodiac to history, mythology and astronomy. Some of the astronomy is rather difficult to grasp and I would refer the interested reader to some of the books listed bellow.
Books and Journals
Aratus ‘Phaenomena’, translated by G.R. Mair, 1955. Loeb Classical Library No.129
A. Gurshtein, ‘When the zodiac climbed into the sky’, Sky & Telescope October 1995 p.28-33.
A. Gurshtein, ‘In search of the first constellations’, Sky & Telescope June 1997 p.46-50.
A. Gurshtein, ‘Prehistory of zodiac dating: the three strata of upper paleolithic constellations’, Vistas in Astronomy 1995(39) p.347-362.
David Hasenauer, ‘What’s your sign?’ Sky & Telescope June 1998 p.10.
E.C. Krupp, ‘Throwing the Bull’ (Taurus), Sky & Telescope April 1997 p.68-9.
Ptolemy ‘Almagest’, translated by G.J. Toomer, Duckworth, 1984 (excellent English translation of Ptolemy’s great work).
Walker, Christopher (ed.) ‘Astronomy before the Telescope’, British Museum, 1996 (contains series of chapters on history of astronomy, good sections on the astronomy of other cultures)