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 Psalm 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 Psalm 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 Psalm 89:46-52 that God would return, and would remember his covenant, and would bless David and his people.

As the moon, and as a faithful witness in heaven - That is, as long as the sun and moon shall endure, as long as time shall last, his kingdom shall last among men. The moon appears to be termed a faithful witness here, because by her particularly time is measured. Her decrease and increase are especially observed by every nation, and by these time is generally estimated, especially among the eastern nations. So many moons is a man old; so many moons since such an event happened; and even their years are reckoned by lunations. This is the case with the Mohammedans to the present day. Or the rainbow may be intended; that sign which God has established in the cloud; that faithful witness of his that the earth shall no more be destroyed by water. As long therefore as the sun, the moon, and the rainbow appear in the heavens, so long shall the spiritual David reign, and his seed prosper and increase.
Selah - It is confirmed; it shall not fail.

It shall be established for ever as the moon, and [as] a faithful (c) witness in heaven. Selah.
(c) As long as the Sun and Moon endure, they will be witnesses to me of this promise.

It shall be established for ever as the moon,.... Either Christ's seed, or throne, which comes to much the same sense; for by both are meant his church and people, his kingdom and interest in the world; the moon is as perpetual as the sun, and is used as elsewhere to signify the continuance of the people, church, and interest of Christ, Psalm 72:5, for though the moon has its spots, and is changeable, sometimes in the full, and sometimes in the decline, yet always is, and always continues, and ever will; and so though the people of God have their spots and imperfections, and are sometimes on the decline in the frames and dispositions of their minds, in the exercise of grace, in their spirituality, liveliness, and zeal, and in their walk and conversation in the church and world; yet they shall abide and persevere to the end; and though the church may be like the moon in the wane, be declining as to numbers, gifts, and graces, yet it shall continue and be established; it is sometimes indeed in a fluctuating state, and is not always in the same place, but is removed from one country to another; yet it always is somewhere, even though in the wilderness, and ere long will be established on the top of the mountains, and be no more a tabernacle that shall be taken down; see Psalm 48:8.
and as a faithful witness in heaven: or "in the sky or cloud" (b); some understand this of the moon, others of both sun and moon; but it seems best to interpret it of something distinct from either, even of the rainbow, which though it does not always appear in the clouds, yet it has appeared at times, and does and will unto the end of the world; and be a faithful and an everlasting token and witness of the covenant of God made with all creatures, that he will no more destroy the world by a flood, Genesis 9:12, and is an emblem of the covenant of grace, and of the continuance, perpetuity, and immutability of it; see Isaiah 54:9.
Selah. See Gill on Psalm 3:2.
(b) "in aethere", Montanus, Tigurine version, Vatablus; "in superiore nube", Junius & Tremellius; "in nubibus", Gejerus.

It shall . . . moon . . . heaven--literally, "As the moon, and the witness in the sky is sure, that is, the moon."

A witness - The rainbow, which is God's faithful witness, a token of God's everlasting covenant between God and every living creature for perpetual generations, Genesis 9:12, Genesis 9:16.