Found, it's Many Moons by James Thurber

[loracarol]

I wish I could remember more about this book, but I really can't: all I remember is that there was a princess who was sad when the moon disappeared, and she got sick/began to waste away, wanting the moon to come back. One day the court jester(?) asks her how big the moon is, and the girl is like "it's no bigger then my fingernail" because when she lifts her thumb up to the heavens, that covers the moon, so the King(?) gives his daughter a necklace with a tiny golden (silver?) moon on it, and they live happily ever after.

I know that is super vague, I'm so sorry! D:

Crossposted to whatwasthatbook