Isaac Watts opens with the best-known idea in Psalm 19, that the sky is a kind of language, and within four lines he quietly ranks it second. The stars show God's "wisdom," but "when our eyes behold thy word, / We read thy name in fairer lines." The verbs make the point. The heavens *declare*, but we *read*. Watts turns the cosmos into a text, then says there is a better one. His central claim is that nature praises God loudly but incompletely, that scripture supplies what the stars cannot, and that this fuller message should travel as far as sunlight does.
The second stanza spells out the division of labour. The "rolling sun, the changing light, / And night and day" only "thy power confess", as if they were witnesses admitting a fact. The "blest volume," by contrast, "Reveals thy justice and thy grace." Power is something you can take in by looking up. Justice and grace are moral qualities, and they need words. The image of the book also gives "lines" a double meaning, since there are lines of stars and lines of verse, and the poem prefers the second kind.
The form is plain and sturdy. There are five quatrains of four-beat, mostly iambic lines, rhymed ABAB, a shape that suits a congregation singing together. A few rhymes are loose. "Light" and "writ" are only half-rhymes, and so are "confess" and "grace". The clean ones, like "Lord" and "word" or "rest" and "blest," land with more weight because of that. Watts also leans on small inversions ("thy power confess," "thy truth has run") to keep the rhymes in place without losing the rhythm.
The sun runs through the whole poem. It rolls in stanza two, joins the moon and stars in stanza three, and by stanza four it has become a measuring stick: Christ will have "all the nations blest, / That see the light or feel the sun." Anywhere the sun reaches becomes the target for the gospel. Nature's range sets the ambition for scripture's range. Watts reinforces this with verbs of motion. The heavens "never stand," the truth begins its "race," it "touched and glanced on every land," and the gospel will not "rest / Till through the world thy truth has run." "Glanced" is a nice choice, because it makes truth behave like a ray skimming across the earth. Against all this movement, the word "rest" is held back to say what the gospel will not do.
Only in the final stanza does darkness appear. Night in stanza two was neutral, just part of the sky's routine, but now we hear of "the dark world." The poem also shifts from description to prayer: "Great Sun of righteousness, arise, / Bless the dark world with heavenly light." Calling Christ the "Sun of righteousness" echoes a biblical phrase, and the sound invites a pun on "Son." The sun image, which started as evidence of God's power, becomes a name for God's saving presence. The poem ends in short, balanced, nearly proverbial phrases: "Thy gospel makes the simple wise; / Thy laws are pure, thy judgments right." The words are almost all single syllables, which makes them easy to sing and hard to forget.
This is where the poem's significance lies. Watts is remembered as a major writer of English hymns, and this text shows why. He takes an ancient psalm, which speaks of God's law, and recasts it in Christian terms, bringing in "Christ" and "gospel," which the original does not name. He also uses simple language to carry a large argument that runs from the universe, to the book, to the whole world, to a prayer. The reader is led from looking up at the night sky to looking outward at everyone else under it.