Who discovers the moon letters on the map?

In "The Hobbit" by J.R.R. Tolkien, Bilbo Baggins discovers the moon letters on the map. These letters, which reveal important information about the secret entrance to the Lonely Mountain, can only be seen when the moon shines on the map during a specific phase. Bilbo's discovery of the moon letters is a pivotal moment, guiding the dwarves on their quest to reclaim their homeland.