THE MILKY WAY

When observing the constellations outdoors at night, you will notice, particularly if the sky is transparent, that a band of faint milky light extends right across the heavens. Its position in the sky will depend on the time of night and the season of the year. In winter, during the early evening, it will be directly overhead, while in the late summer evenings it will be lying low down near the northern horizon.

This band of light extends over more than one-tenth of the visible heavens and in fact represents the galaxy of stars to which the Sun and its attendant planets belong. Most of the stars are located within a horizontal plane, but they are so far distant that we cannot see them as individual points of light with the naked eye. The milky appearance is simply the effect of myriads of faint stars merged together to form a nebulous mist.

However, even with opera glasses and binoculars the milky light can be resolved into innumerable stars, and long-exposure photographs reveal a magnificent structure of star clouds, containing so many individual members that the human eye cannot possibly count them by normal methods. The Milky Way is inclined to the celestial equator by about 63°, and intersects it in the constellations of Monoceros and Aquila. Its width is irregular, in parts only 3°-4° wide, in others 12°-16°. Beginning in Scorpius and travelling northwards, it traverses Sagittarius, Scutum, Aquila, Sagitta, Vulpecula, Cygnus, Lacerta, and projects an 'arm' into Cepheus. It passes through Perseus, Auriga, between Gemini and Orion, then Monoceros and Canis Major. In the southern skies it extends across Puppis, Pyxis, Vela, Carina, Crux, Musca, Centaurus, Lupus, Circinus, Norma, Ara and back into Scorpius.

In the night skies surrounding modern urban developments the Milky Way can rarely be seen at its best. Only in the pitch-black, transparent skies of the tropics do we see it as the ancients once did before air pollution obscured our modern skies. Aratos in his classic poem said: 'that shining wheel, men call it Milk. To the ancient this band of milky light was a great puzzle. The Greeks called it the Galaxy (from gala, milk). To Aristotle, noted for his vivid unscientific imagery, it was, a vast mass of arid vapours which takes fire from glowing trees, above the region of ether and far below that of the planets. Yet to the more scientific Democritus in 460 в с, 'it is the lustre of several small stars which are very near together - an excellent guess indeed at its true nature. In early Akkadian times it was connected with the mythological idea of a Great Sky Serpent, or a Snake River of sparkling dust. The later Arabs also knew it as 'the River', and in China it was 'the Celestial River' or 'Silver River'. It was known as the Path of the Ghosts' in Norse mythology, the route which the fallen warriors took on their journey to Valhalla. In one story of classical Greek folk-lore the Milky Way was marked out by the corn ears dropped by Isis in his flight from Typhon. To the North American Indians, it was 'the Path of Souls', and French peasants nicknamed it 'the Road of St Jacques de Compostelle'.

It was not until Galileo turned to it with his newly constructed telescope that its true nature was confirmed, and he later wrote in his classical work Sidereus Nuntius: '. . . it was nothing else but a mass of innumerable stars planted together in clusters.' Nowadays, we know a great deal more about its physical make-up. We know that the irregular structural features we observe are only due in part to the non-uniform distribution of stars contained within it, but that also a significant element is the presence of an absorbing medium in the form of dark nebulae, or 'coal-sacks', probably consisting of a mixture of various dusts and ices. We know that the Milky Way rotates, and that the Sun takes 200 million years to complete one revolution. We know of places where the stars are created, and we know, for the most part, which celestial objects belong to it, and which are located beyond, in deeper space, at distances truly unimaginable to the mind of man.