<P> The visibility of celestial objects in the night sky is affected by light pollution . The presence of the Moon in the night sky has historically hindered astronomical observation by increasing the amount of ambient lighting . With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky . Special filters and modifications to light fixtures can help to alleviate this problem, but for the best seeing both professional and amateur optical astronomers seek viewing sites located far from major urban areas . </P> <P> The fact that the sky is not completely dark at night, even in the absence of moonlight and city lights, can be easily observed, since if the sky were absolutely dark, one would not be able to see the silhouette of an object against the sky . </P> <P> The intensity of the sky varies greatly over the day and the primary cause differs as well . During daytime when the sun is above the horizon direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light . In twilight, the period of time between sunset and sunrise, the situation is more complicated and a further differentiation is required . Twilight is divided in three segments according to how far the sun is below the horizon in segments of 6 ° . </P> <P> After sunset the civil twilight sets in, and ends when the sun drops more than 6 ° below the horizon . This is followed by the nautical twilight, when the sun reaches heights of - 6 ° and - 12 °, after which comes the astronomical twilight defined as the period from - 12 ° to - 18 ° . When the sun drops more than 18 ° below the horizon the sky generally attains its minimum brightness . </P>

What are the stars in the night sky made of