INTRODUCTION TO LIGHT POLLUTION I - sky glow

For starters, it is very important that we thoroughly understand how Light pollution affects us and also others in our daily life.

A survey by the International Dark Sky Association shows that every 3 out of 5 people who resides in cosmopolitan cities such as New York, Shanghai, Tokyo, Hong Kong and the crux is that, these people are not even aware of it! Or, they just dismiss it as a trivial affair.

Therefore this page will be dedicated to explaining exactly what Light pollution is about, what causes it, how to calculate light, how fast it travels.

1. What is Light Pollution?
Light pollution is basically: Excess obstrusive man-made light which causes illumination of the night sky with artificial light against the natural background radiation.
Usually classified into Sky Glow/Urban Sky glow, glare, clutter, light-trespass and over-illumination
1. Sky Glow


Sky glow refers to the "glow" effect that can be seen over populated areas. It is the combination of all light reflected from all sources of illumination escaping up into the sky, including light that are badly-directed, being redirected by the atmosphere back toward the ground.

This redirection is due to the wavelength of the light when the air is very clear (with very little aerosols). "Rayleigh scattering" dominates in such clear air, making the sky appear blue in the daytime. When there is significant aerosol which is typical of most modern conditions that are moe or less polluted.

The redirected light has less dependence on wavelength, making a whiter daytime sky. Due to this Rayleigh effect, and because of the eye's increased sensitivity to white or blue-rich light sources when adapted to very low light levels (Purkinje effect/ Dark adaptation), white or blue-rich light contributes significantly more to sky-glow than an equal amount of yellow light.



Sky glow is of particular irritation to astronomers, as it reduces contrast in the night sky to the extent where it may even become impossible to see any but the brightest stars. Taking away our "starry starry nights".
This pair of pictures shows us the effects of skyglow. The first picture (left) with its starry starry sky is Ontario on 14/08/03 the night of the East Coast USA Blackout. The second picture (right) is also Ontario on 15/08/03 the night after the EC USA Blackout where the lights are back on again and there werent any stars visible.