If one looks at the sky on a clear, moonless night, thebrightest objects one sees are likely to be the planets Venus,Mars, Jupiter, and Saturn¹. There will also be a very largenumber of stars, which are just like our own sun but muchfarther from us. Some of these fixed² stars do, in fact, appearto change very slightly their positions relative to each other asearth orbits around the sun: they are not really fixed at all!
This is because they are comparatively near to us. As the earthgoes round the sun, we see them from different positionsagainst the background of more distant stars. This is fortunate,because it enables us to measure directly the distance of thesestars from us: the nearer they are, the more they appear tomove. The nearest star, called Proxima Centauri, is found to beabout four light-years away (the light from it takes about fouryears to reach earth), or about twenty-three million millionmiles. Most of the other stars that are visible to the naked eyelie within a few hundred light-years of us. Our sun, forcomparison, is a mere³ light-minutes away! The visible starsappear spread all over the night sky, but are particularlyconcentrated in one band, which we call the Milky⁴ Way. Aslong ago as 1750, some astronomers⁶ were suggesting that theappearance of the Milky Way could be explained if most of thevisible stars lie in a single disklike configuration⁷, one example ofwhat we now call a spiral galaxy⁹. Only a few decades later, theastronomer Sir William Herschel confirmed this idea bypainstakingly cataloging the positions and distances of vastnumbers of stars. Even so, the idea gained complete acceptanceonly early this century.
Our modern picture of the universe dates back to only 1924,when the American astronomer⁵ Edwin Hubble demonstratedthat ours was not the only galaxy. There were in fact manyothers, with vast

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