Charles H. Spurgeon on Psalms 89:37

It shall be established for ever as the moon. The kingdom may wax and wane to mortal eyes, but it shall still abide as long as the moon walks in her silver beauty. And as a faithful witness in heavens. The most stable part of the universe is selected as a type of Messiah's kingdom, and both sun and moon are made to be symbols of its long endurance. Whatever else there is in the sky which faithfully witnesses to the unbending course of nature is also called upon to be a sign of the Lord's truth. When heaven and earth witness, and the Lord himself swears, there remains no excuse for doubting, and faith joyfully reposes in confident expectation.