Well this certainly feels like it’s set in the post-apocalyptic world depicted in Owari no Sekai Kara.
Seems to me like the man set out on a journey to change the world, as implied by his desire to ‘rip the thick dense cloud away’. For some reason he didn’t want to or couldn’t take the woman with him, so he left her behind. A long, long time has passed since he left, and now the woman sees something like a bird or a plane as a nostalgic scene, hoping in her heart that it was his airplane. But it seems like a defeatist sort of hope. She has already accepted that she will never see him again, perhaps that he’s even dead. She hopes he’s still out there, but she’s already accepted that they’ll never meet again. I guess that’s why she wishes that she could’ve joined him, so she wouldn’t have to suffer this pain.
I’m curious what people think about the ‘winter constellations’ part though, not sure what to make of that.