The ultimate example is how the sun prevents us from seeing the stars during the day. Essentially, our air is slightly blue and you can see this if you shine a light through it. Human light pollution is much dimmer than the sun but there’s still an effect.
Another factor is the limited range of brightnesses that our eyes can see at any one time. In a dark environment, chemical changes in our eye increase sensitivity, allowing us to see much dimmer things. This can take 20 to 30 minutes but a single glance at a bright light will ruin the effect and your dark adaptation has to start again. Even if nearby lights in the city didn’t brighten the sky, they’d still stop your eyes from adjusting fully to see dim objects.
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