From the uttermost part of the earth to the uttermost part of heaven (ἀπ' ἄκρου γῆς ἕως ἄκρου οὐρανοῦ)
From the outermost border of the earth, conceived as a fiat surface, to where the outermost border of the heaven sets a limit to the earth. Compare Mat 24:31. Mark's expression is more poetical.