The heavens declare the glory of God - They announce, proclaim, make known his glory. The word heavens here refers to the material heavens as they appear to the eye - the region of the sun, moon, and stars. The Hebrew word is used in the Scriptures uniformly in the plural number, though in our common translation the singular number is often used. Genesis 1:1, Genesis 1:8-9, Genesis 1:14, Genesis 1:17, Genesis 1:20; Genesis 6:17; Genesis 7:11, Genesis 7:19, Genesis 7:23; et soepe. The plural, however, is often retained, but without any special reason why it should be retained in one place rather than in another. Genesis 2:1, Genesis 2:4; Deuteronomy 10:14; Ezra 9:6; Psalms 2:4; Psalms 8:1, Psalms 8:3; Psalms 18:13. The original idea may have been that there was one heaven above another - one in which the sun was placed, another in which the moon was placed, then the planets, the fixed stars, etc. Above all was supposed to be the place where God dwells. The word glory here means that which constitutes the glory or honor of God - his wisdom, power, skill, faithfulness, benevolence, as seen in the starry worlds above us, the silent, but solemn movements by day and by night. The idea is, that these convey to the mind a true impression of the greatness and majesty of God. The reference here is to these heavens as they appear to the naked eye, and as they are observed by all men. It may be added that the impression is far more solemn and grand when we take into the estimate the disclosures of the modern astronomy, and when we look at the heavens, not merely by the naked eye, but through the revelations of the telescope.
And the firmament - See the note at Daniel 12:3. The word rendered firmament - רקיע râqı̂ya‛, means properly “an expanse” - that which is spread out - and is applied to the heavens as they appear to be spread out or expanded above us. The word occurs elsewhere in the following places, and is always rendered “firmament” in our common version, Genesis 1:6, Genesis 1:7 (twice), Genesis 1:8, Genesis 1:14, Genesis 1:15, Genesis 1:17, Genesis 1:20; Psalms 150:1; Ezekiel 1:22-23, Ezekiel 1:25-26; Ezekiel 10:1; Daniel 12:3. The word “firmament” - that which is firm or fixed - is taken from the word used by the translators of the Septuagint, στερέωμα stereōma, from the idea that the heavens above us are a solid concave. In the Scriptures the stars are represented as placed in that expanse, so that if it should be rolled together as a tent is rolled up, they would fall down to the earth. See the note at Isaiah 34:4. The reference in the passage before us is to the heavens as they appear to be spread out over our heads, and in which the stars are fixed.
Showeth his handywork - The heavens make known the work of his hands. The idea is that God had made those heavens by his own hands, and that the firmament, thus adorned with sun, and moon, and stars, showed the wisdom and skill with which it was done. Compare Psalms 8:3.