The first half of Psalm 19 is a meditation on Genesis 1. Heavens and sky refer to all of creation. The “heavens” is the atmosphere in this case, and the skies are the firmament from Gen 1:7 (רָקִיעַ). In the worldview of the Ancient Near east, the “vault of heaven” was a kind of “beaten metal plate” on which the sun, moon, and stars moved. The Psalmist uses this commonly understood description of the world and argues it reveals something important about God.
The heavens both “declare” and “proclaim.” These common verbs refer to speech acts, although “declare” can have the sense of making a formal record, often a written record. Usually this word has the sense of a pronouncement of some important information, even a “report.” It is the verb, for example, used for the report from the twelve spies in Numbers. Both verbs are participles, emphasizing the ongoing nature of this testimony: the heavens are continually declaring God’s glory. Coupled with the “day and night” of verse two, the writer is clear this revelation is constant and ongoing.
“Pouring forth speech” is a vivid metaphor of rapid speech. The verb (נבע) refers to “bubbling” or “gushing” water. Think of the way an excited five-year old tries to relate a story, words gush from the kid as fast as they can talk (usually one long sentence you can’t follow anyway!) Creation is a constant flow of information about God.
The content of all this constant speech a revelation of knowledge. This knowledge certainly contains facts, but there is more to it than a series of propositions since biblical knowledge leads to proper response to God. In Prov 9:10, for example, parallels the fear of the Lord as the beginning of wisdom of God with knowledge of the Holy One.
This testimony goes out to the whole world even though there is no speech (19:3-4). In theological terms, this Psalm is talking about creation as general revelation from God. Humans can know some things about God from observing nature, his existence and power, for example. By analogy, there are universal symbols virtually every culture knows and understands (poison is a skull and crossbones, for example). You may not know what the poison is in the bottle, but anything with that symbol is understood as dangerous, especially if there are red letters and exclamation points.
This revelation is so clear there is no one who can escape God’s self-revelation, he is like the sun in the sky, a bridegroom proudly going up from a wedding or a warrior charging into battle. “Just as the sun dominates the daytime sky, so too does Torah dominate human life” (Craigie, Psalms 1-50; WBC, 184). This is similar to Paul’s point in Romans 1:18-25. God clearly reveals his existence and some of his attributes in order to draw people to himself, but humans suppress this knowledge and worship created things rather than the creator.
If God reveals himself so clearly in creation, why do people twist or reject that revelation?