By - askmekaigainiki
First of all -- please don't assume that there is logic in prepositions. There often is some kind of logic, but... Sometimes they are deeply confusing.
Secondly -- I have never heard someone say the sun/moon/stars was "under" a cloud before. In my dialect (Canadian English) I am more likely to hear "behind a cloud."
It's the same for me (in the UK). It would definitely be behind a cloud.
I would say “behind” here in the US, too.
It makes sense in a weird way, but I agree - it does sound awkward.
My guess is they didn't want to use passive voice like "what if the sun was blocked by clouds" but wanted to keep "sun" as the subject.
"What if clouds blocked the sun?" would sound a lot better, I think.
I think "under" here is being used more like "under the effect" rather than literally under something.