Although it is fairly clearly explained that Thorin’s main goal is to reclaim his homeland, the kingdom under the Lonely Mountain known as Erebor, The Hobbit: An Unexpected Journey takes the time to explain the map he inherited from his grandfather that will help him accomplish this. . They take the map to Elrond in Rivendell, who accidentally discovers runes on it that “write with the moon”. Written by Thorin’s grandfather, Thror, under a “silvery broad crescent”, they can only be read under exactly the same conditions. Fortunately, such a moon fell on that very night, the mother of all coincidences.
With this amazing luck, the runes show that on “Durin’s Day” (essentially the last day of autumn in the dwarven calendar), the last light of sunset will illuminate the keyhole of the secret door while you too stand. at the gray stone when the thrush knocks. How Thorin’s grandfather knew that the owner of the map could distinguish one gray stone from another, or that a thrush would knock at the right moment, is not clear. It’s also a bit confusing why Elrond had to decipher this information when the map was received from Thorin’s family. To be fair, it’s been a hectic couple of centuries of wandering for all the Dwarves of Erebor, so this information could have been scattered to the wind.