What is the comparison of his throne to the moon?
Radak: Just like the moon is sometimes dark and sometimes illuminates, so will be his seed. When they will be good, they will illuminate; when they will be evil, they will be dark. In any case the kingship will not cease, just like the moon does not cease, not when it is dark nor when it is illuminating.
Malbim: It is established also in the lower world, like the moon is established forever. Even when its light darkens at the end of the month, it immediately renews its light. So, whenever David's throne sinks and totally darkens, it returns to shine anew like initially.
Why does it say "v'Ed ba'Shachak Ne'eman Selah"?
Rashi: The moon and sun are witnesses for him. As long as they exist, his kingship will endure - "Im Taferu Es Brisi ha'Yom v'Es Brisi ha'Laylah" (Yirmeyah 33:20), so My Bris with David Avdi will be annulled [only if the sun and moon cease].
Radak: The moon is a faithful witness in Shachak (a high level of Shamayim) about David's seed. Hashem said that his seed will be forever, like the moon.
Malbim: Before it explained how the Chesed and Emunah, with which Hashem made a Bris with David, are compared to the Chesed and Emunah guarded in the existence of the world at large. David's throne resembles the sun and moon in the world at large. The author laments, the witness in Shachak is faithful and enduring. This has a twin intent. The simple meaning is that the witness that Hashem prepared for the Bris, the sun and moon, are in Shachak. The author also laments, that the Chesed and Emunah is fulfilled for the witnesses in Shachak (the sun and moon), but the Chesed and Emunah promised to Beis David is not fulfilled, like it will explain.
