Psalms 89:37 - Matthew Poole's English Annotations on the Holy Bible

Whereby he understands, either, first, The moon, last mentioned, to which this clause may be added rather than to the sun, to imply that as the moon, though subject to eclipses and frequent and manifold changes, yet doth constantly and perpetually remain in heaven, as a witness of my covenant of the night, as it is called, Jeremiah 33:20; so shall the house and kingdom of David continue for ever, not withstanding all the changes and calamities which it may undergo. Or, secondly, The rainbow, which though in itself it be unstable and transient, and doth but seldom appear, which learned men object against this opinion, yet in Scripture is mentioned as God's faithful and perpetual witness, being called a token of God's everlasting covenant between God and every living creature for perpetual generations, Genesis 9:12,16. And although it do not always appear to us, neither do the sun or moon do so, yet its appearances are doubtless very frequent in one or other part of the world, and will be repeated from time to time to the end of the world. Add to this, that the word here rendered heaven, may as well be rendered the cloud or clouds, as it is used Deuteronomy 33:26 Job 35:5, Job 36:28 Psalms 18:12, Psalms 77:17 78:23 Proverbs 3:20 Isaiah 45:8. And so the place being thus translated, and as the faithful witness in the cloud or clouds, doth plainly point us to the rainbow.