On A clear crisp night, away from the yellow glow of city lights, it is easy to make out a broad pale ribbon stretching across the heavens; a stream of brightness that stands out, albeit faintly, against the black background of the sky. Over the centuries, it has been the subject of much inventive speculation.
In the folklore of Lapland, the Milky Way represents the track to be followed by migratory birds on their long journey south. In African countries it was believed to be the "spine" of the night, holding up the darkness lest it should crash in fragments at our feet. And early Greek astronomers thought the Milky Way was the joining together, or "seam", of the two halves of the celestial globe.
For the Tartars of the Caucasus, the Milky Way marked the trail of a mythical thief who leaves behind him copious scatterings from a bale of stolen straw. And in virtually all cultures, it is often thought of as the path of souls departing for the after-life, a path that leads to the land inhabited by the dead.
But the ancient Romans had the most romantic tale of all. They identified the Milky Way as the highway across the heavens to the great palace of Jupiter. Towards the end of the so-called Golden Age, Jupiter, the chief god, was greatly distressed at the depraved state of humanity, so their story went. He was so angry that he summoned all the gods to council, and the road they took to his palace can still be seen to stretch across the darkened sky.
It was a decisive meeting; it was decided to destroy the earth and all that lived upon it. Accordingly, Jupiter took up his thunderbolt and was about to launch it on the unsuspecting world when it occurred to him that such a major conflagration might well set fire to heaven too, so he changed his plan and sent a flood instead.
When Galileo pointed his new telescope at this luminous band of light 450 years ago, he recognised it immediately for what it was. It consisted of the fused light of countless stars, too small and close together to be distinguished individually.
We now know that our galaxy is disc-shaped, rather like a gigantic Catherine Wheel, and that when we look at the Milky Way we are looking along the disc where the density of stars is greatest. In other directions, we are looking across the "thinness" of the galaxy, and we thus see fewer stars.