Shadow is
Baldr, the son of Odin. The Poetic Edda and the Prose Edda, compiled in Iceland in the 13th century, but based on much older Old Norse poetry, contain numerous references to the death of Baldr as both a great tragedy to the Æsir and a harbinger of Ragnarök.
Shadow dies in Wednesday’s place on the world ash tree Yggdrasil, in the eastern USA, as a squirrel whispers Ragnarok in his ear and gives him water in a walnut shell, as a deliberate retelling of the Norse trope. The Norns have made the rope ladders on which Shadow is hung.
Part of this framing of the American story against the Norse mythology is to present an American everyman, who due to bad luck has nothing, who follows his fate towards a confrontation between the real powers represented by the different myths. With the idea of the Gods of new technology as in conflict with the forgotten pagan Gods of mythology, Gaiman is framing an apocalyptic clash between old and new. However, Baldur/Shadow finds himself a reconciling and redeeming contact between old and new, as the Minnesota grifters Odin and Loki are shown up as lacking balance.
The ladders of Yggdrasil made by the Norns have a striking astrophysical parallel. Valhalla has 540 gates. Seeing these gates as years, this image maps to a braid of three ladders up a cosmic tree, with rungs every sixty years. Interweaved like DNA spirals, our solar system has just such a system of three encircling ladders whose rungs are formed every 59.8 years. The conjunctions of Jupiter and Saturn, the period also central to Mayan time reckoning in the Katun, are every 19.9 years, and every third conjunction is a third of a sign (10°) further around than the one sixty years before. After three rungs (179 years), Jupiter and Saturn meet up again with Neptune, one sign past their last meeting, an event that never occurred in the twentieth century but will happen on Jupiter's next meeting with Neptune in 14 years. Of course this ‘unknown unknown’ planet Neptune was unknown before its modern discovery, but it adds a neat twist to the old parallels of Jupiter and Saturn with the myths of the structure of time. The three rope ladders of Baldr on Yggdrasil, leading to Valhalla, are actually the cyclic interaction of the planets Jupiter and Saturn.
Edited to add: The three ladders formed by the twenty year Jupiter Saturn conjunctions have rungs separated by 60 years – eg 1960 to 2020. The ladder can be modelled and has an angle from the vertical of 10° as noted above. Nine of these 60 year Jupiter Saturn rungs = 9 x 60 years = 540 years. In
American Gods, the Norns weave three rope ladders, each with nine rungs, around the world tree to string up Shadow/Baldr, matching the model of the solar system’s main two planets. Hence the three nine rung rope ladders of Yggdrasil add to a number of years, 540, in their Jupiter-Saturn incarnation, which matches the number of 540 gates of the Norse Heaven Valhalla. The hanging on the tree is what Wednesday points towards in their mead and spit deal when he says Shadow must do his vigil.