Because 'sits above the circle of the earth' Be honest, what picture comes to your mind?
Well, the obvious picture - the same that was intended, and that no doubt came to the Hebrew readers back when this was written: ol' YHWH sitting up there on his throne, above the stars and sun and such on the dome of the "firmament." And that dome was "spread out like a tent" over the circle = disk of the earth. Look at Daniel 4:10-11 :
...I saw, and behold a tree in the midst of the earth, and the height thereof was great. The tree grew, and was strong, and the height thereof reached unto heaven, and the sight thereof to the end of all the earth:
How do we do that on a sphere? You are putting words into Isaiah's quill that just weren't there. He meant "disk," because he thought the worrld was an immovable disk:
1 Chronicles 16:30: He has fixed the earth firm, immovable.
Psalm 93:1: Thou hast fixed the earth immovable and firm ...
Psalm 96:10: He has fixed the earth firm, immovable ...
Psalm 104:5: Thou didst fix the earth on its foundation so that it never can be shaken.
Isaiah 45:18: ...who made the earth and fashioned it, and himself fixed it fast...
as did his contemporaries. The early Church fathers had enough Greek "book learnin'" to know it was really a sphere, and start the apologetics to rationalize that error away. The "immovable" held on until Galileo's day and a little after, and then the spinmeisters respun those verses, too, or at least quit reading them as often.