Albert Barnes Commentary Psalms 89:37

"It shall be established for ever as the moon, And [as] the faithful witness in the sky. Selah" — Psalms 89:37 (ASV)

It shall be established forever as the moon - As long as the moon will endure. The heavenly bodies are the most permanent objects that we know; and therefore, they 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 also be said of the sun and the stars; but the beauty of the image is increased by the attention being fixed on 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 God had taken; such was 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 events were occurring which made it appear as if God had forgotten them; that the manifest prosperity and favor implied in the promise was not present; but that a series of calamities had occurred which it was difficult to reconcile with these solemn pledges.

On this basis, he prays (Psalms 89:46–52) that God would return, and would remember his covenant, and would bless David and his people.