the profound solitude or lifelong remorse of a person (originally referring to Chang'e or a faithful widow) facing the vast universe alone

嫦娥应悔偷灵药，碧海青天夜夜心。

the green sea and the blue sky, describing a scene where the sea and sky appear to merge into one shade

站在甲板上，展现在眼前的是一幅碧海青天的壮丽画卷。