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Thrór's Map is a map that proved important in the retaking of Erebor.
As was traditional among the Dwarves, it was drawn with East at the top of the map and showed the Lonely Mountain of Erebor, the Running River that flowed from it, and the lands about. It also contained mention of a secret entrance to the Mountain, and (written in secret Moon-letters) instructions as to how it might be opened.
In his old age, Thrór set out to wander the lands of Middle-earth, but before departing he gave the heirlooms of his house to his son Thráin, including his Map. Years later, Thráin too set out into Middle-earth, seeking to recover Erebor himself, and taking with him the Map to guide his way. He was captured in the Wild by the servants of Sauron, but Sauron captured Thráin for one of the Seven Ring of Power he bore, and so overlooked the Map he carried too. This proved to be a serious oversight, because Gandalf secretly entered Sauron's lair at Dol Guldur while Thráin was a prisoner there, and managed to escape with the Map and the key that opened the secret door.
Using Thrór's Map, his grandson Thorin was able to lead a small group of Dwarves (and one Hobbit, a certain Bilbo Baggins) on his own quest to refound the ancient Dwarf-realm. Through many adventures, they were able to recover Erebor and bring about the destruction of Smaug, dealing Sauron a serious reverse.
The small text on the left, below the hand, reads.
the door an
d three may
The moon-letters read by Elrond are visible in the sketch by Tolkien and read:
Stand by the grey s
tone when the thrush kn
ocks and the setting s
un with the last light
of Durin's day will sh
ine upon the keyhole
 The illustration of the Map
"Thrór's Map" is reproduced in facsimile in The Hobbit (and as plate No. 86 in J.R.R. Tolkien: Artist & Illustrator). The map was drawn by J.R.R. Tolkien, and a preliminary version of the map appears as plate No. 85 in J.R.R. Tolkien: Artist & Illustrator.
 Possible Date of the Creation of the Map
The Dragon Smaug attacked and destroyed the Kingdom Under the Mountain in T.A. 2770. Thrór went into exile with his kin until, old and desperate, he went to Moria and died at the hands of Azog in T.A. 2790. When Elrond read the Moon-letters in 2941 he stated that they had to have been written under a moon in the same phase on a midsummer night in some past year.
Astronomically, the moon repeats its phases about every nineteen years. From 2770 to 2941 there are 171 years, which is exactly nine time nineteen years, which means that the moon was in the right phase on midsummer night in 2941, 2922, 2903, 2884, 2865, 2846, 2827, 2808, 2789, and 2770. Since Thrór died in 2790 he could only have made his map in 2770 or 2789 (provided he made it after Smaug's attack). It is not known when in 2770 Smaug descended upon the Lonely Mountain; if after midsummer's night then Thrór would not have had any reason to draw his map. It appears that 2789 is the most likely year for the creation of this document.[source?]
- ↑ Wayne G. Hammond and Christina Scull, J.R.R. Tolkien: Artist & Illustrator, pp. 90-5
- ↑ J.R.R. Tolkien; Douglas A. Anderson, (ed.), (2002) The Annotated Hobbit: Revised and Expanded Edition, p. 399
- ↑ 3.0 3.1 J.R.R. Tolkien, The Lord of the Rings, Appendix B, "The Third Age"
- ↑ J.R.R. Tolkien, The Lord of the Rings, Appendix A, "Durin's Folk"
- ↑ J.R.R. Tolkien, The Hobbit, "A Short Rest"