Nature: October 2007 Archive Page

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. 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). 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.

The stars have not become dimmer; rather, the Earth has become vastly brighter, so that celestial objects are harder to see. 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. 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.