Dust and water-vapour absorb and scatter the starlight, making it difficult to see fainter objects.

At its worst, clouds (or smoke and other pollutants) are the ultimate obstruction.

The dust and vapour will also scatter any light from the ground back to your eyes, e.g. from street lights, building lights, traffic, etc.

This is called "light pollution", which makes the sky appear brighter and makes the viewing problem even worse. The term "transparency" describes how clear the sky is. To give yourself the best chance of viewing the heavens, try to get as far away from built-up areas as possible.

If you live in a built-up area where street lighing causes a problem to your night-time viewing, you should try contacting your local council to see if they will do something about a nuisance street lamp. Often a polite letter requesting part of a lamp be painted black, or a hood fixed to it, will get a worthwhile response. Certainly councils have been known to help in situations where bright light shining into a bedroom disrupts the occupants sleep.


Next section: Astronomy: What is Seeing?

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