Did you ever think of the sky as being hollow? I like the image of its domed roundness as the inside of a mussel shell.
‘At midnight the western sky was pale blue and hollow like a mussel-shell on the seashore.’
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 56
Photo credit: stux at pixabay.com