Why is it here НА небі while on another sentence there was літак В небі? Is there any difference in the use of the preposition?
Both "у" and "на" are used with небо, but in different situations. You know, if we imagine sky to be some kind of a shell above the Earth, than we can also imagine that stars and Moon are on its surface. So in Ukrainian we say місяць на небі and зорі на небі. We also use на when speaking about sky in the meaning of heaven: Бог на небі (God), янголи на небі (angels). For planes and other things flying in the sky we say "у": птахи у небі (birds), літак у небі