Charles H. Spurgeon on Psalms 89:37

And as a faithful witness in heaven. (New Translation) And as the rainbow's faithful sign. The rainbow is not expressly mentioned in the original, which speaks only of "the faithful witness in heaven." Some commentators understand the "witness" thus mentioned to be no other than the moon itself. I prefer, however, the interpretation that fixes it on the rainbow, which God after the deluge appointed as a "sign" or "witness" of his mercy in Christ. Ge 9:12-17. Conformably to this appointment, the Jews, when they behold the rainbow, are said to bless God, who remembers his covenant and is faithful to his promise. And the tradition of this its designation to proclaim comfort to mankind was strong among the heathens: for, according to the mythology of the Greeks, the "rainbow" was the daughter of "wonder", "a sign to mortal men", and regarded, upon its appearance, as a messenger of the celestial deities. Thus Homer with remarkable conformity to the Scripture account speaks of the "rainbow", which "Jove hath set in the cloud, a sign to men." —Richard Mant.