13 I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth. 14 When I bring clouds over the earth and the bow is seen in the clouds, 15 I will remember my covenant that is between me and you and every living creature of all flesh. And the waters shall never again become a flood to destroy all flesh. 16 When the bow is in the clouds, I will see it and remember the everlasting covenant between God and every living creature of all flesh that is on the earth.” (Genesis 9) [ESV]
Bow is קֶשֶׁת which means bow for shooting arrows. As there is no word for "rainbow" in Hebrew, the "bow" in the passage is understood to be a "rainbow."
Greek has separate words for bow (τόξον) and for rainbow ( ἶρις - [cf. Revelation 4:3, 10:1]). Despite having a word which means "rainbow" the LXX translator(s) used "bow:"
13 I set my bow in the cloud, and it shall be for a sign of covenant between me and the earth. 14 And it shall be when I gather clouds upon the earth, that my bow shall be seen in the cloud. 15 And I will remember my covenant, which is between me and you, and between every living soul in all flesh, and there shall no longer be water for a deluge, so as to blot out all flesh. 16 And my bow shall be in the cloud, and I will look to remember the everlasting covenant between me and the earth, and between living soul in all flesh, which is upon the earth. (LXX-Genesis 9)
13 τὸ τόξον μου τίθημι ἐν τῇ νεφέλῃ καὶ ἔσται εἰς σημεῗον διαθήκης ἀνὰ μέσον ἐμοῦ καὶ τῆς γῆς 14 καὶ ἔσται ἐν τῷ συννεφεῗν με νεφέλας ἐπὶ τὴν γῆν ὀφθήσεται τὸ τόξον μου ἐν τῇ νεφέλῃ 15 καὶ μνησθήσομαι τῆς διαθήκης μου ἥ ἐστιν ἀνὰ μέσον ἐμοῦ καὶ ὑμῶν καὶ ἀνὰ μέσον πάσης ψυχῆς ζώσης ἐν πάσῃ σαρκί καὶ οὐκ ἔσται ἔτι τὸ ὕδωρ εἰς κατακλυσμὸν ὥστε ἐξαλεῗψαι πᾶσαν σάρκα 16 καὶ ἔσται τὸ τόξον μου ἐν τῇ νεφέλῃ καὶ ὄψομαι τοῦ μνησθῆναι διαθήκην αἰώνιον ἀνὰ μέσον ἐμοῦ καὶ ἀνὰ μέσον πάσης ψυχῆς ζώσης ἐν πάσῃ σαρκί ἥ ἐστιν ἐπὶ τῆς γῆς
Obviously "bow" is literally correct, but this suggests the sign of the "bow" was something other than a rainbow.
Was God's sign other than a rainbow? If not, what does the failure to describe the sign as a rainbow convey?