Lord Byron wrote this poem inspired by a cousin named Anne. If you look deep into Byron's biography, you know three things:
1. He had sort of incestuous fantasies and passions 2. He was bipolar, and 3. He was extremely passionate.
This cousin, Anne, according to lore used to wear a black gown that was accented with glittery details, just like the night is glittered by stars, which is one of the similes we find.
He does a play of words with lots of poetic devices to try and explain the complexity of this woman's beauty. She is daylight and nightlight, she is beauty and innocence, she is depth and breadth.
He goes overboard trying to dig inside the mind and soul of this incorruptible woman (the type of woman that would make him want to corrupt, of course). The way he balances the meeting together of all her amazing qualities is a play on verses to make the reader feel the pull and presence that this woman's face instills in him.