I think Nasa has lived with Tsukasa for much, much, much, much longer than he realizes. I think he's a being that will die, and be reincarnated once more and Tsukasa is immortal. That's why Tsukasa agreed to marry Nasa right off the bat. She knows him already.
It's also why he might be good at stars and constellations, not only did he study them, he might even know them because of his past on the moon with Tsukasa, I could be wrong on that one.
It's also why he's dreaming of something "nostalgic" here. He's been there before, and he probably lived there with her.