Complex sentence: The fact that the sky is not completely dark at night can be easily observed since if the sky is (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky.
Split into simpler sentences: The fact that the sky is not completely dark at night can be easily observed. When the sky is (in the absence of moon and city lights) absolutely dark, one would not be able to see the silhouette of an object against the sky.