Why does the sky behind the blue sky become plural?

Sky is singular when it refers to a part of the sky; Refers to the vast sky or over a certain area, and clouds are often used in the plural; For "weather, climate, customs", the plural form is often used at this time; Sometimes the plural form is also commonly used in literary language.

Who dares to let the sun and moon shine in the new sky?

Dare to teach the sun and the moon to change the sky for sacrifice and ambition.