Albert Barnes Commentary Psalms 19:2

"Day unto day uttereth speech, And night unto night showeth knowledge." — Psalms 19:2 (ASV)

Day to day - One day to another; or, each successive day. The day that is passing away proclaims the lesson it had to convey from the movements of the heavens about God; and in this way, the knowledge of God accumulates as time moves on. Each day has its own lesson regarding the wisdom, power, and goodness of God, and that lesson is conveyed from one day to another. There is a perpetual testimony thus given to the wisdom and power of the Great Creator.

utters speech - The word here rendered 'utters' properly means to pour forth; to pour forth copiously like a fountain (Proverbs 1:23; Proverbs 15:2, 28). Hence, the word means to utter, to declare. The word "speech" properly means "a word;" and then, "a lesson;" or "that which speech conveys." The idea is that the successive days thus impart instruction or convey lessons about God. The day does this by the returning light, by the steady and sublime movement of the sun in the heavens, and by all the disclosures made by the light of the sun in its journeys.

And night to night shows knowledge - Knowledge respecting God. Each successive night does this. It is done by the stars in their courses: in their order, their numbers, their ranks, their changes of position, their rising, and their setting. There are as many lessons conveyed to people about the greatness and majesty of God by the silent movements of each night as there are by the light of successive days. In a similar way, there may be as many lessons conveyed to the soul about God in the dark night of affliction and adversity as there are when the sun of prosperity shines upon us.