In Galileo’s time, nighttime skies all over the world would have merited the darkest Bortle ranking, Class 1. They have no comparison to work against.” “People will sometimes come up from the city and call me and say, ‘John, I’ve found this fabulous dark site, it’s totally black, you can’t imagine how good it is.’ So I’ll go and have a look, but it’s always poor. “One of the problems I was addressing was that younger amateur astronomers, especially east of the Mississippi, had never seen a dark sky at all,” he told me recently. Bortle, a retired Westchester County fire chief and a monthly columnist for Sky & Telescope. The scale, composed of nine points, was devised in 2001 by John E. Amateur astronomers sometimes classify nighttime darkness on the Bortle Dark-Sky Scale, which is based on a number of criteria, among them “limiting magnitude,” or the brightness of the faintest celestial objects that are visible without magnification. Today, a person standing on the observation deck of the Empire State Building on a cloudless night would be unable to discern much more than the moon, the brighter planets, and a handful of very bright stars-less than one per cent of what Galileo would have been able to see without a telescope. 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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