The sky appears as blue only when the sunlight enters the atmosphere. The sunlight is made up of a spectrum of colours, known as VIBGYOR or rainbow colours. As soon as the light enters the atmosphere, the colours all scatter as they pass from different particles in the air. The colour in the light with the shorter wavelength spreads more. Violet colour having the shortest wavelength is scattered all over the sky but because human eyes are sensitive and cannot observe the violet colour. We observe the sky as blue, which is next on the list or pattern of the colour spectrum.
On the contrary, the sky appears black at night because of the absence of sunlight. Without sunlight the sky masks itself with the blackness of space. The moon that is a natural satellite shines because it reflects sunlight, which falls over it and shines when the earth is not receiving the light.
