astronomy & light pollution If you were staring to the sky in a big city and electricity is turned off in a big area around you, would you be seeing the sky with little light pollution instantly?
 A: No, your eyes would need to adjust to the decrease in light before you would see really well. Also, it is now very difficult to have zero light pollution. Mt Palomar experiences light pollution and it is at least 2 miles from light sources.
A: If you include car lights and similar in what is turned off, then in a big city such as London, which has a radius of about 10 miles, there would be very little light pollution, instantaneously.   However, there would still be a lot of atmospheric pollution, and so it wouldn't be possible to see stars as faint as can be seen from, say, the top of a mountain.
A: On the Bortle Dark-Sky Scale the sky brightness would change from "Class 9: Inner-city sky." to perhaps "Class 6: Bright suburban sky." In Western Europe you won't get much better than Class 6 skies even if you are 200 km away from big cities, because the light of these distant cities will still make the sky slightly gray which has the effect of drowning out faint nebulae. So, while you will see many more stars, I'm pretty sure you won't be able to spot the galaxy M33 with the naked eye, even if you try hard.
