In Milton: A Poem by William Blake by Kay Parkhurst Easson and Roger R. Easson we receive an explanation of the concave earth which results from the centrality of man.
"Blake describes the earth as concave, as cavern of 'labyrinthine intricacy'; this earth is the human skull or body which, if observed from without, is convex but, if observed from within, is concave. Similarly Blake describes the sky as an 'immortal Tent' ; man 'standing on his own roof, or in his garden' sees such concave space as his universe; he sees it from within to without. Because this is true, Blake insists that the astronomer's model of the earth as 'a Globe rolling through Voidness' is a delusion, for such would be the model of an earth separated from its perceiver. In a concave model, however, all the earth, the sky, the universe are within man's immediate perception.
This view of the physical world attests that Blake has returned the physical body of the individual, and the soul within it, to the center of his cosmos as the medieval world view had done before him. This is the Vitruvian man, the man in the center of a circle proscribed by the extentions of his limbs. Here, however, Blake's universe is proscribed not by a concepual framework of geography and terrain, but only by the limits of and individual's perception... What Blake asserts, then, is that it is our interaction with perceived objects which is the essential element of reality.
Just as the Vitruvian man stands in the center of his circle, we stand in a perceptual concavity which is much like a great sensory balloon. Blake calls this concavity the Mundane Egg."
Jerusalem, Plate 71, (E 225) "What is Above is Within, for every-thing in Eternity is translucent: The Circumference is Within: Without, is formed the Selfish Center And the Circumference still expands going forward to Eternity. And the Center has Eternal States! these States we now explore." Jerusalem, Plate 71, (E 225) "All were his Friends & their Sons & Daughters intermarry in Beulah For all are Men in Eternity. Rivers Mountains Cities Villages, All are Human & when you enter into their Bosoms you walk In Heavens & Earths; as in your own Bosom you bear your Heaven And Earth, & all you behold, tho it appears Without it is Within In your Imagination of which this World of Mortality is but a Shadow."
Jerusalem, Plate 72, (E 227) "All the Nations Peoples & Tongues throughout all the Earth And the Four Gates of Los surround the Universe Within and Without; & whatever is visible in the Vegetable Earth, the same Is visible in the Mundane Shell; reversd in mountain & vale And a Son of Eden was set over each Daughter of Beulah to guard In Albions Tomb the wondrous Creation: & the Four-fold Gate Towards Beulah is to the South[.] Fenelon, Guion, Teresa, Whitefield & Hervey, guard that Gate; with all the gentle Souls Who guide the great Wine-press of Love; Four precious stones that Gate:"