What is Walt Wiltman's poem "When I Heard the learn'd Astronomer" about?

Whitman is saying that the true way to understand nature is not scientific but intuitive and mystical. The poet can feel and understand the processes of nature when he is experiencing them, but listening to people lecture about them merely makes him "tired and sick."

Summary of the poem: The speaker of this poem describes listening to a learned astronomer lecture. He sees proofs and figures in columns before him, as well as charts and diagrams that he is supposed to analyze mathematically. He gets sick of the mathematics and charts, and goes out and looks at the night sky.