I feel like it's related to the fear of falling? The night sky in an open field would make me uneasy if I focused on it, like I was going to fall upwards. Same feeling as being near a steep ledge. The worst was climbing firetowers though, nngh... Being at the top of one, feeling it swaying, just felt so *unanchored*.
I used to climb them fairly often with my family, but when I tried again recently I didn't manage it. I probably could have, but I had nothing to prove and I was extremely uncomfortable halfway up. The occasional wooden "step" being missing really didn't inspire confidence either.
But yeah, I've got the same feeling by looking up at the sky in a clearing. Where all I can really see is emptiness and distant stars, no anchoring objects. I used to dream about floating in vast voids, too... Sorta falling, sorta just being overwhelmed by an abstract feeling of *scale*, like huge numbers or long times. Was kinda nauseating.
Definitely beautiful, but alien and overwhelming to one's ego. The sort of fear common in Lovecraft stories, I suppose.
Edit: I wonder if I can find a panorama I took from a bare rocky outcrop that was surrounded on 3 sides by drop. (Not that I want to scare anyone). They say not to look down, but I couldn't bare to look *up* - I had to keep the stone beneath me in view. The sight of the ground far below wasn't bad. But if I looked up, and didn't have my fingers on cold steady rock, it was like I was adrift in the sky.