Air pollution has made the atmosphere less transparent and more reflective, and high levels of terrestrial illumination have washed out the stars overhead-a phenomenon called “sky glow.” Anyone who has flown across the country on a clear night has seen the landscape ablaze with artificial lights, especially in urban areas. The stars have not become dimmer rather, the Earth has become vastly brighter, so that celestial objects are harder to see. Today, by contrast, most Americans are unable to see the Milky Way in the sky above the place where they live, and those who can see it are sometimes baffled by its name. It truly resembled a streak of spilled liquid-our word “galaxy” comes from the Greek for milk-and it was so bright that it cast shadows on the ground (as did Jupiter and Venus). It may seem strange that this last observation could have surprised anyone, but in Galileo’s time people assumed that the Milky Way must be some kind of continuous substance. His homemade telescopes had less magnifying and resolving power than most beginners’ telescopes sold today, yet with them he made astonishing discoveries: that the moon has mountains and other topographical features that Jupiter is orbited by satellites, which he called planets and that the Milky Way is made up of individual stars. ![]() ![]() In 1610, Galileo Galilei published a small book describing astronomical observations that he had made of the skies above Padua. ![]() The word “galaxy” comes from the Greek for milk. A time-exposure photograph of the Milky Way over the New Mexico desert.
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