The sky is part of the atmosphere or space that can be seen from the earth"s surface, birds, insects, aircraft and was considered a kite flying in the sky. There are several reasons why the sky is difficult to define?
In the daytime the sky appears blue because air is caused the scattering of sunlight. Not because there are blue objects above the earth. For this reason, so that it is difficult to define that the sky is anything. The sky was sometimes define a region of the earth"s atmosphere with gas tight, at night, the sky appears as a black. Or sometimes lined with stars. But if we say that all we see is sky. It will become that the sky is the universe, which is different from the first meaning when we see during the day.