Understanding the Night Sky

The bortle scale 9 which is the inner city skies is different from bortle 4-5 only due to the light pollution. The big cities with hi-rise buildings, flyovers and big roads, shopping malls, street lights and more such places use electricity to light up the streets and buildings which emits a lot of light towards the skies. With durst and pollution, the city forms and envelop in the sky which scatters the light as it interact with the particles, creating and artificial blanket of light altering the amount of starlight which can reach us. This blanket restricts the light coming from fainter stars. Only the bright star light can reach us and that is also much fainter than the actual. This is called as light pollution. The excess light does not allow our eyes to be adjusted to darkness and hence we miss out on the faint star light reaching us in any case.

When we are at a dark sky location, the sky is dark and the surrounding are also very dark. This allows our pupil to expand in order to adjust our vision in the darkness. This is a natural tendency of human eyes to adjust with the change in light conditions. The pupil expands slowly and hence in few minutes after dark, we can see things better.

Take an example of movie theatre. Suppose you are 5 minute late to the movie, and when you enter the theatre, the movie has started, the lights are off and its absolutely dark. You entered the theatre through the open area which is well lit and hence your pupil are contracted to suit the light conditions. Now as you enter, you can barely see any thing and as you start walking towards your seat, you find it very difficult to see things initially. You ofter gets imbalanced, sometimes steps in the wrong place but after a lot of struggle, you find your seat. Now after 10-15 minutes, you see another person entering the theatre and now you see him struggling the same way as you did but only you can see things better. You can find the stairs, see the seats etc. This is because the last 10-15 minutes you spent inside the theatre has made your eyes adaptable to the dark conditions inside the theatre. This adaptability to see things in the dark is called Dark Adaptation.

As a bright light is flashed towards us, the pupil contracts immediately as a safety mechanism in place to avoid any damage to our nerves and cells in the eyes. So it takes minutes to adapt to darkness whereas fraction of seconds to adjust to light. That is why we prefer to move to dark skies.