Zitat:
と is often used without a suitable verb; with a verb of emotion, it might mean it's being said in that way. In this case, I think it means the quoted statement is being thought. Possibly whispered, but we don't really know that, we just might think that of someone that's looking up at the sky and wishing (praying?) for something to come true.
It could also be taken to mean 'looking up at the sky -as though to say- "If it comes true... "'
I'm less inclined to this interpretation only because we're already inside the head of the character with knowing about this wish held in his heart. If the rest of the sentence were purely external description it'd be different.
Zitat:
he easiest way to think about it is that the phrase has an implied verb after と, in this case likely: 叶えばと(思って)空を見上げる. This is a common device (example: 彼は、また明日と帰った = 彼は、また明日と言って帰った).
叶えばと is describing the manner in which 人は空を見上げる. In a poetic sentence such as this one, the explicit inclusion of 思う (or 言う or any other suitable verb) would be limiting whereas the implicit use could include any number of additional nuances.
The condition form of 叶う→叶えば gives it a sense of wondering, hope, and longing.
Also kann と auch verwendet werden, wenn das zugehörige Verb „verschwiegen“ wird? So etwas scheint im Japanischen ja zumindest recht oft vorzukommen.