This beautiful little excerpt from Leaves of Grass is a perfect example of the difference between Neoclassicism and Romanticism. The narrator, presumably Walt himself, is listening to a lecture on the “science” of astronomy, separated from the stars themselves because he is indoors, being subjected to a mathematical, scientific "proof" of the actual phenomena. Soon becoming “tired and sick,” he leaves the lecture hall, goes out under the “mystical, moist night air,” and gazes “in perfect silence” at the stars. The Romantic notion is to get back in touch with the beauty all around us in Nature, and to respond to it emotionally rather than “naming” and “charting” and taxonomizing everything is a cognitive way; the subtle irony of the phrases “in perfect silence” and “much applause” demonstrate the difference between the “noise” of humans, and the "perfect silence" available to the viewer of the real thing. Like all of Whitman's poetry, it is in blank verse, but has a subtle natural rhythm to its pace--reinforcing the poem's essence.
While written in free verse, the pattern that the reader immediately notes is the repetition of the word "When" at the beginning of the first four lines--a technique known as anaphora. At this point in the poem, Whitman is talking about the astronomer and the science of stars, the realm that is structured, just like the beginning lines. When Whitman switches that pattern, he states, "How soon accountable I became tired and sick," and the poem finishes without pattern. When Whitman becomes "tired and sick" and the structure and patterns about the stars and nature, he realizes that he needs to be out in nature enjoying it instead of stuck inside listening to a "learn'd" person talk about it.