Charles Ellicott 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)

And as a faithful witness in heaven. —Rather, and there is a faithful witness in heaven, which the parallelism shows to be the moon, just mentioned. The moon was to the Jews—as to the ancients generally—the “arbiter of festivals,” and the festivals were signs of the covenant, consequently that luminary might well be called “a witness in heaven.”