Harry had never even imagined such a strange and splendid place. It was lit by thousands and thousands of candles that were floating in midair over four long tables, where the rest of the students were sitting. These tables were laid with glittering golden plates and goblets. At the top of the hall was another long table where the teachers were sitting. Professor McGonagall led the first years up here, so that they came to a halt in a line facing the other students, with the teachers behind them. The hundreds of faces staring at them looked like pale lanterns in the flickering candlelight. Dotted here and there among the students, the ghosts shone misty silver. Mainly to avoid all the staring eyes, Harry looked upward and saw a velvety black ceiling dotted with stars. He heard
Hermione whisper, "It's bewitched to look like [sky 1] the sky outside. I read about it in Hogwarts, A History."
It was hard to believe there was a ceiling there at all, and that the Great Hall didn't simply open on to [heaven 2] the heavens.
(Harry Potter and the Sorcerer's Stone)
Merlin nodded: this one would summon him to London decades later when hellfire fell from [sky 2] the skies. (from COCA, The Queen and the Cambion by Richard Bowes)
There is/are the sky/skies or heaven/heavens above the Great Hall. I remember reading plural mountains is because mountains cannot be separated as a clear-cut unit, for they are connected, in CGEL (I can’t find from what page, now). So I thought [sky 1] is the very near sky just outside the hall, which cannot be separated, for the seer and the sky is close enough, the borders with the other skies cannot be - imaginarily -lined between them.
In this respect, [sky 2] is the ones that are spread out to only goblins knowing where; [heaven 2] opens to likewise.
Is this what the singular, plural means?