Although it is explained quite clearly that Thorin’s general quest is to reclaim his homeland, the kingdom beneath the lonely mountain known as Erebor, The Hobbit: An Unexpected Journey rushes the explanation of the map he inherited from his grandfather , which will help him to achieve this. They bring the map to Elrond in Rivendell, who accidentally discovers “moon writing” runes on it. Written by Thorin’s grandfather Thror under a “broad silver crescent”, they can only be read under exactly the same conditions. Luckily in the mother of all coincidences there is just such a moon on the very same night.
With this amazing luck, the runes reveal 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 a secret door as long as you “stand” by the gray stone when the thrush knocks. ” How Thorin’s grandfather knew that the bearer of the map could tell one gray stone from another, or that a thrush would knock at just the right moment, is unclear. It’s also a bit confusing why Elrond had to decipher this information when the map was from Thorin’s own family. To be fair, it’s been a hectic few centuries of vagrancy for all the dwarves of Erebor, so that information may have been thrown to the wind.