。
When night fell, she gazed at the moon.
Literal
Night [to-に] become-[when-と] she [topic-は] moon-[honorific-お~様] [object-を] gazed.
お月様 uses the affectionate お~様 framing for the moon — common in children's stories and fairy tales, giving it a personified, cherished quality. ながめる (眺める) means 'to gaze at wistfully,' more contemplative than 見る. ~になると marks a natural temporal transition.