Ἀρχόμενος: ἄρχομαι was the vox propria for the opening invocation of a hymn, cf. Arat. Phaen. 1, “Ἐκ Διὸς ἀρχώμεσθα”, Hom. Il. 9. 97, “ἐν σοὶ μὲν λήξω, σέο δ᾽ ἄρξομαι.” It has here a special appropriateness, as the oracle of Apollo brought to pass the quest of the golden fleece.
κλέα φωτῶν: cf. Il. 9. 189, “ἀειδε δ᾽ ἄρα κλέα ἀνδρῶν”. Since the α is shortened in Homer in the phrase κλέα ἀνδρῶν, Ap. regards himself as justified in treating the α as short generally. We find the same expression in h. Hom. 32. 18, “σέο δ᾽ ἀρχόμενος κλέα φωτῶν ᾁσομαι ἡμιθέων”.