Psalms 89:37 - Ellicott's Commentary On The Whole Bible

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 (see Psalms 81:3) 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.”