by Paul Murdin
The word ‘planet’ comes from the Greek word for ‘wanderer’, because originally any celestial objects that were not ‘fixed’ stars were considered to be planets. The Sun and Moon were thought to be planets and their motions could, with careful attention and the keeping of records, be predicted. It seemed reasonable to assume that the same was true of Mercury, Venus, Mars, Jupiter and Saturn, which were visible to the naked eye as bright lights roaming across the heavens. To many ancient observers of the skies it seemed that, if they could understand and predict the motions of the seven planets, they might uncover the deepest mysteries of the cosmos.
We have a tangible connection with one of the first known observers of the Moon, 25,000 years ago. He or she was one of a community known as the Ishango people who, until they were dispersed or wiped out by a volcanic eruption, lived, fished and farmed along the shores of what we now call Lake Edward, one of the sources of the Nile in central equatorial Africa. This person scratched markings in groups of twenty-nine on the bone handle of a knife or chisel. These markings have changing sizes that, according to anthropologist Alexander Marshack, represent the varying phases of the Moon throughout six months of the lunar cycle. There are gaps in the markings that seem to represent cloudy nights when the Moon was not seen.
We know nothing more about the maker of the Ishango bone – the earliest surviving lunar calendar. Perhaps the carver was a hunter or traveller keeping track of a long journey, perhaps a woman keeping track of her menstrual cycle.
Across the continents the erection of monuments and, where they have survived, the evidence of ancient myths and legends reveal the many different ways in which prehistoric men and women made sense of the cosmos and its relationship to their lives. It is unlikely that these peoples made precise astronomical observations, even at an elaborate primitive observatory like Stonehenge, but the keeping of calendars that tracked the solar and lunar cycles required the first civilizations to develop more complex patterns of thought and communication. Modern physicists use advanced mathematics to explain the operation of the Universe. It was the observation of the heavens that encouraged humankind to take the earliest steps towards mathematical thought.
The first known systematic observations of the planets were recorded on Babylonian cuneiform tablets, starting from about 1700 BCE. In the Neo-Assyrian period (911–612 BCE) astronomers regularly recorded the motions of the planets and, based on what they saw, gave astrological forecasts to the king. Astronomers of the Achaemenid kingdom kept astronomical diaries of their observations, which they used to make predictions about affairs of state, the level of the River Euphrates, and the price of goods such as barley, dates, mustard, sesame and wool.
Alexander the Great’s conquests in the East in the fourth century BCE brought the Babylonians’ detailed astronomical records to the attention of ancient Greece, where philosophers such as Thales of Miletus, Pythagoras, Plato and Aristotle had debated the nature of the Universe and used geometry to explain planetary motions. The Babylonians’ example encouraged Greek astronomers to base their speculations upon more exact observations of the stars and planets.
The Greeks assumed that the Earth, which was not a planet, lay stationary at the centre of the Universe – after all, the Earth doesn’t rock about as if it is moving, nor do the positions of the stars change as if we are viewing them from a succession of positions along an orbit. This was the ‘geocentric’ theory of the planets and it is associated with the name of the astronomer Claudius Ptolemaeus, more often known as Ptolemy.
Ptolemy worked at or near Alexandria in Egypt during the middle decades of the second century CE. By 147 CE he had developed the geocentric theory to a sophisticated level, described in outline in a public inscription and presented in full in a large treatise entitled the Almagest, written in Greek in the second century CE, but now known only through Arabic versions that were translated into Latin in the Middle Ages (its title, meaning ‘the greatest’, comes from an Arabic translation).
The Ptolemaic theory of the Universe held that the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn revolved in a succession of concentric orbits. They orbited around the Earth as it lay motionless at the Universe’s centre. Although astronomers had by this time introduced the idea of orbits, the picture that was commonly in mind was that the planets were mounted on a series of hollow crystal spheres. Outside the sphere of the most distant planet, Saturn, was the sphere of the stars – because at any time of the night the stars seem to be studded on a hemisphere above us and this sphere appears to rotate around the Earth. (Navigators still use the convention of the celestial sphere to calculate the positions of the stars as seen from ships.) Beyond the sphere of the stars was an unseen sphere – the ‘primum mobile’ (‘first mover’) – which was the primary mechanism that drove the movements of the Universe.
What were the engines that rotated the celestial spheres on their axes? The Christian astronomers who adopted Ptolemy’s theories eventually offered their own answer: eight of Gabriel’s angels pushed the spheres through their rotations.
In its essentials, the geocentric theory of the Universe is the common-sense view of its structure: it is what we feel and see. The true arrangement of the Universe would remain hidden from human perception for nearly 2,000 years.
Stars and Northern Constellations
Our human link with the Ice Age
The whole of mythology could be taken as a sort of projection of the collective unconscious. We can see this most clearly if we look at the heavenly constellations, whose originally chaotic forms are organized through the projection of images. This explains the influence of the stars as asserted by astrologers. These influences are nothing but unconscious, introspective perceptions of the collective unconscious.
Carl Jung, The Structure of the Psyche, 1927
Although the stars are random in position and brightness, they form shapes that people can interpret symbolically, just as they read clouds, tea leaves or entrails. Even before recorded history, people have seen mythical heroes, animals, birds and everyday objects in the patterns of the stars. The names of these ancient constellations are still used by astronomers, and are among the oldest surviving elements of human culture.
One of the most ancient constellations is Ursa Major, which was identified as a bear by northern peoples in both North America and Eurasia, and must therefore have pre-dated the disappearance of the land-bridge joining Alaska and Siberia around 15,000 years ago. The concept of the constellation as a bear then spread southward into the Middle East and the eastern Mediterranean, even though bears had disappeared from these lands as the glaciers retreated at the end of the last Ice Age. The Pointers of Ursa Major help to locate the North (pole) Star in the sky, and for this reason the Great Bear is often the first constellation that people in the Northern Hemisphere learn. Ursa Major symbolizes the northern regions and features on the flags of Alaska and the Cherokee nation.
Forty-five constellations were described in the Phaenomena, a poem written in about 275 BCE by the Macedonian Greek Aratus of Soli. The poem was itself based on earlier work, now lost, by Eudoxus, a Greek astronomer and mathematician of the fourth century BCE. Eudoxus pored over old manuscripts in libraries in Egypt, which recorded original constellations dating back to the Babylonian civilizations of Mesopotamia. These manuscripts, too, are lost (some in the fires that destroyed the Library of Alexandria by the fifth century CE), but the constellations they recorded survived in Greek literary culture. By the age of the poet Homer, the Babylonian constellations had become interwoven with Greek mythology, a process that was virtually complete by the third century BCE.
Most of the ancient constellations acquired their present Latin names in the first and second centuries as Greek mythology became absorbed into Roman culture and Latin translations of Greek texts like Phaenomena appeared. Representations of the constellation figures appeared: the oldest surviving representation is the statue known as the Farnese Atlas. Sentenced by Z
eus to hold up the celestial sphere, the Titan Atlas struggles under its weight in this second-century Roman marble copy of a Hellenistic sculpture. The globe shows forty-one of the classical Greek constellations listed by Aratus and is the oldest surviving picture of them. There has been speculation that the constellations in this depiction are based on the lost star catalogue by Hipparchus dating from 129 BCE. The Greek-speaking astronomer Ptolemy, who lived in the Roman colony of Alexandria in Egypt, described forty-eight constellations in his Almagest. These descriptions of the constellations formed the basis for the constellations of the present day.
The oldest surviving map of the stars on paper is Chinese, and is known as the Dunhuang Star Chart, dating from about 720 CE. It identified four hundred Chinese asterisms (sub-constellations): they are smaller and more numerous than Western constellations, and not usually related to them, although some are recognizable, such as the Seven Stars of the Northern Dipper (in Chinese, Beidou Qixing), part of Ursa Major. This star chart is one of tens of thousands of scroll books that were discovered in 1908 in a library in the Mogao Buddhist caves near Dunhuang in northwest China, hidden in a room that had been bricked up for a thousand years in anticipation of the imminent arrival of marauders.
In the West, a number of constellations have been added into Ptolemy’s list, filling both the gaps between the Greco-Roman constellations and the large gap in the southern skies that was not visible from Mediterranean latitudes. The Polish astronomer Johannes Hevelius named seven constellations in 1687. They included Lacerta, since, as Hevelius wryly reasoned, only a lizard could wriggle into the small space available, and the Lynx, because the sharp eyes of a lynx were needed to see its faint stars. Some astronomers attempted to honour patrons by naming constellations for them, but only one survives: Hevelius’s constellation Scutum was originally Scutum Sobiescianum, named after Poland’s King John III Sobieski.
Other modern constellations never found widespread favour and were included on some charts but ignored on others. In 1922 the International Astronomical Union (IAU) took charge of the chaotic situation. Under the Belgian astronomer Eugène Delporte, the IAU standardized the constellations to the official modern system, abbreviating some over-elaborate names, rendering many constellations obsolete. There are now eighty-eight recognized constellations with fixed boundaries.
Alongside the official names of the constellations, there are common names in use in various languages. Gemini, for example, is known as the Twins in English, Gémeaux in French, Zwillinge in German and Gemelli in Italian. There are also names for asterisms, such as the Plough, the Big Dipper and Charles’ Wain (all actually naming the same part of Ursa Major) and the Hyades (a star cluster in the constellation Taurus). The Summer Triangle of the northern sky (an equilateral triangle formed by the stars Altair, Deneb and Vega) is an asterism that spans several constellations. Some Australian Aboriginal peoples saw a constellation in the Southern Cross that was not made of stars, but of dark clouds in the shape of an emu.
The zodiacal constellations are the major constellations that the Sun, Moon and planets cross as they travel around the sky. These constellations are of different sizes, but give their names to a system of twelve abstract ‘zodiacal signs’ that divide the sky evenly into sectors that are 30 degrees long and which provide a way of describing the positions of the Sun and planets. When the system was first devised, about 3,000 years ago, there were six zodiacal constellations, all of them representing animals, such as Leo, the Lion – hence the name of ‘zodiac’, a Greek word meaning ‘figured like animals’. Six further constellations were later interpolated, some of them inanimate, such as Libra, the Scales. The twelve zodiacal signs were named after the twelve zodiacal constellations. This location system for planets constitutes the framework used by astrology, which bases its predictions on planetary positions. The zodiacal signs emerged in Babylonian astronomy during the fifth century BCE and travelled to Greece, Egypt, Rome and India. Because the Solar System tilts, due to the phenomenon of ‘precession’, a thirteenth constellation, Ophiuchus, now intrudes into the zodiac, but this has not affected the system of zodiacal signs, and astrology remains largely indifferent to it. The continuing popularity of consulting one’s ‘star signs’ in daily horoscopes attests to the lasting influence of the ancient constellations on the public imagination, even though the prophetic powers of astrology are pseudo-scientific nonsense.
The Milky Way
Path of the gods, souls and pilgrims
He had gauged the southern skies with greater results than even he himself had anticipated. Those unfamiliar constellations which, to the casual beholder, are at most a new arrangement of ordinary points of light, were to this professed astronomer, as to his brethren, a far greater matter. It was below the surface that his material lay. There, in regions revealed only to the instrumental observer, were suns of hybrid kind – fire-fogs, floating nuclei, globes that flew in groups like swarms of bees, and other extraordinary sights – which…turned out to be the beginning of a new series of phenomena instead of the end of an old one.
Thomas Hardy, Two on a Tower, 1882
Our Sun is part of a disc-shaped collection of stars called the Milky Way Galaxy, or just ‘the Galaxy’. The Sun lies in the plane of the disc. When we look at the sky along the direction of this plane, we see large numbers of stars, massed into a filmy band. This is the Milky Way, which has the appearance of a milk-like stream of light, although it is not actually a band or a stream, but a disc of stars seen edge-on.
The earliest surviving written description of the Milky Way is in Ptolemy’s Almagest. ‘The Milky Way is not simply a circle,’ he wrote, ‘but a zone having almost the colour of milk, whence its name. It is not regular and ordered but different in width, colour, density and position; in one part it is double.’ Of course, the very word ‘milky’ suggested the literal mythological explanation, a favourite scene of painters, Tintoretto among others, that the Milky Way was the milk of the goddess Juno (plate I). The fourth-century BCE Greek scientist and philosopher Aristotle made the first scientific discussion about the Milky Way in his Meteorologica, written around 350 BCE, and many Western ideas about the origin or interpretation of the Milky Way can be traced back to this source, in which he discussed and classified all the theories of his time.
Because we live centrally in the plane of the disc of our galaxy, the direction of greatest number of stars in the Milky Way stretches in a great circle around the sky. The Milky Way thus has the appearance of an arch, bridge or road across the night sky, conspicuous from dark locations both in summer and in winter evenings. Near the polar regions, the summer nights are short and twilit so Swedes, for example, can scarcely see the Milky Way during the summer and view it only in the long nights of the winter season. Swedes call the Milky Way Vintergatan – ‘Winter Street’.
The Milky Way is bifurcated in some of its sections, giving it the appearance of a meandering river split by dark, elongated islets. This is caused by a thin, crinkled layer of dust, which from Earth is seen edge-on, silhouetted against the Milky Way and hiding an irregular zone of light originating from the broader disc of the stars beyond. Because of this visual effect, in Arab lands the Milky Way was known as Al Nahr (‘the River’), but the name was never adopted into Western languages because of the potential for confusion with a long meandering constellation with the name of a river, Eridanus.
The Milky Way has been named, too, after specific roads. English names for the Milky Way include Watling Street, after the Roman road built from Chester to London and on to Dover, and Walsingham Way, after the road from London to the Virgin’s shrine in Norfolk. Local people thought that the Milky Way pointed to the shrine. In Spain the Milky Way is sometimes called El Camino de Santiago. This is the pilgrim’s track to Santiago de Compostela in northern Spain. Both pilgrims’ ways were thronged with travellers, as the Milky Way is with stars.
In many cultures the Milky Way symbolized the journey of the soul into the afterl
ife. Coupling the belief that the stars represent souls with the idea that the Milky Way was a road, the Romans described the Milky Way as the path to the Seats of the Heroes, ‘Heroum Sedes’, walked by the departing souls of illustrious men. In Timaeus (c. 360 BCE), Plato described how
after having thus framed the universe, [the supreme divinity] allotted to it souls equal in number to the stars, inserting each in each…and he declared also, that after living well for the time appointed to him, each one should once more return to the habitation of his associate star, and spend a blessed and suitable existence.
The similarity of the pale appearance of the Milky Way to grey ash produced another class of explanations: that it was the scorched path of the Sun or, in classical mythology, the disastrous route travelled by Phaeton, son of the sun-god Helios, when he lost control of his father’s chariot, which carried the Sun across the sky. Jupiter put a stop to this potential disaster by throwing a thunderbolt, causing Phaeton to hurtle in flames into the river Eridanus. The final theory mentioned by Aristotle was that the Milky Way was some sort of manufacturing imperfection, like the seam of a metal casting or the sewed seam around a leather-covered ball.
The true explanation for the Milky Way’s ‘milk-like’ appearance is that it is made up of many stars, which are too faint and too close to each other to be viewed individually. This was first conjectured in the fifth century BCE by the Greek philosopher Democritus and finally proved by Galileo with his telescope in 1610.
