Praise ye the Lord - See the notes at Psalms 146:1.
Praise God in his sanctuary - His holy place; the place where he dwells. The allusion here is, probably, to the temple, the place of his abode on earth.
Praise him in the firmament of his power - The whole expression is equivalent to earth and heaven; Praise him on earth; praise him in heaven. The word rendered firmament is the same which is used in Genesis 1:6. It properly means an expanse - a thing spread out. The verb from which the word is derived means to beat; then, to beat out - that is, to spread out by beating, as gold is; and then, simply to spread out, to expand. Compare Psalms 136:6; Isaiah 42:5; Isaiah 44:24. In Syriac the word means to make firm; but this idea is not necessarily in the Hebrew word. The idea of a firmament as something firm is derived from the Septuagint - in Genesis 1:6, στερέωμα stereōma - in this place, ἐν στερεώματι en stereōmati. The Hebrew, however, merely means “an expanse” - something spread out, as the heavens seem to us to be “stretched out;” and the call here is on all that dwell above that expanse - in heaven - to unite with those on earth in his praise. It is called “the expanse of his power” because it is in the heavens - in the sun, the moon, the stars - that the power of God seems to be principally displayed.