Here's a couple stabs at it:
(1) It's a neuter plural generalizing phrase, of a sort well-exemplified
elsewhere in the NT: TA EPOURANIA, TA EPIGAIA.
(2) Semitic (Hebrew at least) has a plural (dual?) for Heaven: ha
Shamayim,which seems to be represented at some points in the NT with the
plural hOI OURANOI.
There may be five or six other reasons too. Maybe they will rise to
consciousness and come back to us. ;-)
Carl W. Conrad
Department of Classics, Washington University
One Brookings Drive, St. Louis, MO, USA 63130
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