The ability to separate two objects on the night sky is determined by the resolving power of the eye, or instruments being used to observe the heavens.
A telescope (special lens/mirrors system) increases the apparent visual size of the observed body.
||Atmospheric Effects - Particulates|
Particulates in the atmosphere cause light to be scattered. This has two effects - firstly to diminish the brightness of the stars and secondly to increase local scattered light.
||Atmospheric Effect - Heat|
The warmer the atmosphere the more turbulent it becomes. This means that the light path through the atmosphere is changing so objects appear to brighten and fade (twinkle).
||Atmospheric Effects - Light Pollution|
Light given off from local sources (such as street lights) brightens the sky and makes it harder to see fainter objects.