It shall be established forever as the moon - As long as the moon shall endure. The heavenly bodies are the most permanent objects that we know of; and they, therefore, became the emblems of stability and perpetuity. Compare the notes at Psalms 72:7.
And as a faithful witness in heaven - As the witness in heaven, or in the sky, is sure. The reference is to the moon, regarded as a witness for God. What is said here of the moon as an index of his faithfulness, might be said also of the sun and the stars; but the beauty of the image is increased by the attention being fixed to a single object. As the moon is fixed, regular, enduring - so are the promises and purposes of God. Such were the promises made to David; such was the oath which had been taken by God; such the covenant which he had made. The psalmist now proceeds Psalms 89:38-45 to show that this oath and these promises seemed to be disregarded; that there were things occurring which appeared as if God had forgotten them; that there was not that manifest prosperity and favor which was implied in the promise; but that a series of calamities had occurred which it was difficult to reconcile with these solemn pledges. On the ground of this he prays Psalms 89:46-52 that God would return, and would remember his covenant, and would bless David and his people.