Elrond can see the moon letters on Thror's map because the moon is of the correct shape and season.
Explanation:
The fairy-tale fantasy "The Hobbit" by J. R. R. Tolkien is a set of series of stories about wizards and hobbits and elves and many fantastical figures. It details their journey to reclaim a lost kingdom from a dragon and the dark powers.
In the Hobbit, we find the group of 13 men were on their way to the Mountain along with the wizard Gandalf when they approached the Elven kingdom. They also asked the Elven Lord Elron to read the map for them for he can read the moon letters on the map. These moon letters were a method of writing invented by the Dwarves. These provide them the one missing detail on how to get into the mountain. Elron can see the moon letters because of the correct shape and season of the moon.
