Getting transparent about light pollution
A key factor that influences how bad the effects of light pollution are is the haziness of the sky
If you live in an urban or suburban area, the fact that badly directed and excessive artificial light obscures the stars over our towns and cities isn’t going to be news to you. But if you’re just getting into stargazing, it can be useful to explore exactly how this light pollution affects certain kinds of astronomical observing – not least because this can help you prepare for nights when the effects are lessened somewhat. Clouds, of course, can spoil an observing session no matter where you’re observing, but there’s another atmospheric phenomenon that all townbased stargazers will know is hugely important to the clarity of their views: astronomers call it the ‘transparency’.
Atmospheric transparency is a measure of how clear your clear skies are. Hazes or pollution (suspended in the air) can create milky, murky skies and what can be called ‘poor’ transparency. Conversely, crystalclear nights free of these intrusions are said to possess ‘good’ transparency.
The reason why transparency is important to monitor when observing the night sky from urban areas is that hazy skies can elevate the problems associated with light pollution: the particles in the air scatter the glow from below and the result is a brighter night sky, where it’s harder to see fainter stars. Paying attention to transparency levels can help you plan observing sessions. If you can see that the sky is milky and the skyglow is enhanced you can focus your attention on brighter targets like the Moon and planets. When the transparency is good you can go after fainter objects like star clusters.