Spilled milk, a silver river, a path of straw: naming our galaxy
Greeks saw spilled milk in the pale band across the night sky, giving us both Milky Way and the word galaxy. Across East Asia it is the Silver River. Finns and Lithuanians call it the Birds' Path, because migrating birds seem to follow it, and Swedes know it as Winter Street.
The Greek name galaxias kyklos meant milky circle, one of 11 great circles the Greeks traced on the sky, and passed into Latin as via lactea. One myth says Zeus set the infant Heracles at the sleeping Hera's breast to make him immortal, and the milk that sprayed when she pushed him away became the band of light. Chaucer used the English phrase around 1380. Arabs, Persians and Turks speak of a pathway of straw, possibly borrowed from Armenian; Sanskrit tradition names it the Ganges of the sky; and medieval pilgrims steering by it toward Santiago de Compostela called it the Road to Santiago.
What they were all seeing is our own galaxy viewed edge-on from inside. The Milky Way is a barred spiral about 87,400 light-years across but only around 1,000 thick in its arms, holding perhaps 100 to 400 billion stars. The Sun sits some 27,000 light-years from the centre, on the inner edge of the Orion Arm. At the heart lies Sagittarius A*, a black hole of about 4.1 million solar masses, and the oldest stars formed not long after the Big Bang. Dark lanes such as the Great Rift and the Coalsack are clouds of dust hiding stars behind them; the Inca and Aboriginal Australians read those shadows as constellations of their own.
Understanding came slowly. Aristotle thought the glow belonged to Earth's upper atmosphere, a view Olympiodorus the Younger later attacked because the band shows no parallax. Galileo's telescope resolved it into individual stars in 1610, but until the early 1920s most astronomers believed the Milky Way contained every star in existence. After the 1920 Great Debate between Harlow Shapley and Heber Curtis, Edwin Hubble's 1923 observations showed it to be one galaxy among many.
Seeing it now takes effort. The band is faint, needing a sky darker than about 20.2 magnitudes per square arcsecond, and light pollution hides it from more than a third of humanity. Northern observers get their best views from May to August, looking south between Sagittarius and Scorpius, with eyes given half an hour to adapt and no moon in the sky.
Source: Milky Way
