I decided to render τόν οὐρανόν and τήν γῆν as "heaven" and "earth" (without the English definite article) because it seems to me that, in English, "heaven" and "earth" are sufficiently definite, and I liked the flow without the articles. I don't think we really have an indefinite concept, in English, of either "heaven" and "earth," except perhaps as communicated by the plural "the heavens." And both ideas, again in English, strike me as not being qualitative.
Actually, the most difficult translation decision I had to make vis-à-vis Gen. 1.1-5 was how to render ἐπεφέρετο [3 sing imp pass ind ἐπιφέρω, "I bring, I impose, I offer," inter alia]. The NETS renders καὶ πνεῦμα θεοῦ ἐπεφέρετο ἐπάνω τοῦ ὕδατος as, "and a divine wind was being carried along over the water." Had I taken this same route I would have rendered this, "And the Spirit of God was being brought [blown?] upon the water."
The Liddell-Scott lexicon, however, lists as one possible use for the passive voice of ἐπιφέρω "to rush upon or attack, assault" (s.v.). Given the polemical nature of Genesis 1 as a creation account that systematically denies other, polytheistic understandings of the cosmos and its creation, and given the role of "chaos"/"water"/"the deep" (or "abyss") in other creation accounts, it seemed attractive to me that the LXX translators took this opportunity to present the πνεῦμα θεοῦ as doing battle with (and triumphing over; lit. "was bringing attack upon") the water. Hence my translation of ἐπεφέρετο as "was subduing."