John Calvin Commentary Psalms 89:36

"His seed shall endure for ever, And his throne as the sun before me." — Psalms 89:36 (ASV)

His seed shall endure for ever. There now follows the promise that the right of sovereignty shall always remain with the posterity of David. These two things — his offspring and his throne — are conjoined; and by these words the everlasting duration of the kingdom is promised, so that it should never pass to those of a foreign and different lineage.

The sun and the moon are presented as witnesses; for although they are creatures subject to corruption, yet they possess more stability than the earth or air, as the elements, we see, are subject to continual changes. As the whole of this lower world is subject to unceasing agitation and change, there is presented to us a more steadfast state of things in the sun and moon, so that the kingdom of David might not be judged according to the common order of nature.

However, since this royal throne was shaken in the time of Rehoboam, as we have previously remarked, and afterwards broken down and overthrown, it follows that this prophecy cannot be limited to David. For although eventually the outward majesty of this kingdom was brought to an end without hope of being re-established, the sun did not cease to shine by day, nor the moon by night. Accordingly, until we come to Christ, God might seem to be unfaithful to his promises. But in the branch that sprang from the root of Jesse, these words were fulfilled in their fullest sense.