What Romantic ideas does this poem express and how does the structure of the poem reinforce the meaning?
"When I Heard the Learn'd Astronomer" is, in many ways, essentially a poetic expression of Immanuel Kant's ideas, key to an understanding of Romanticism, on the nature of the sublime. Kant drew on the work of Edmund Burke and stipulated that Nature's beauty cannot and should not be quantified; what is beautiful in nature is a "presentation of an indeterminate concept of understanding."This is what the speaker in Whitman's poem experiences: the astronomer, attempting to condense the beauty of nature and the stars into "charts and diagrams," eventually leads the speaker to become "tired and sick."
It is only by experiencing nature for its own sake that the speaker is able to experience its "mystical" qualities, looking up at the stars "in perfect silence." This idea can also be found in, for example, Wordsworth's preface to the Lyrical Ballads, wherein he describes poetry—particularly in relation to the sublime in nature—as "emotion recollected in tranquillity." The theme of the "mystical" elements of nature as divorced from neo-Classical charting and study recurs in Wordsworth's "Tintern Abbey" and "I Wandered Lonely as a Cloud," among others.
The structure of Whitman's poem reinforces its meaning in that the initial four lines, with their repeated "When" and similar sentence forms, contribute to a sense of exhaustion deriving from repetitiveness: these lines become longer and longer without much variance, reflecting Whitman's experience of the schoolroom. In the second half of the poem, this repetition falls away entirely, and the verse becomes, like the speaker, less constrained, freer, and more unpredictable.
Neoclassical poets were very much interested in returning to the antique era of poetry, marked by such epics as The Odyssey and The Iliad. As a result, they were interested in recreating the poetic devices common within the cannon (such as repetition, like anaphora). Whitman nods to the neoclassical tradition in using anaphora within is free verse. He further does so by setting his poem within the rigors of the classroom, where the students are studying "charts and diagrams, to add divide and measure" the stars. These students mirror the neoclassical poets themselves, studying the forms and patterns of classical poetry.
When the anaphora breaks, the poem's speaker moves outside to observe the stars directly, enacting a major tenet of Romanticism: the connection between the individual and nature. Like protestant reformers who did not need the Catholic clergy to commune with God, so too is Whitman's protagonist able to revel in the sublimity of nature directly. True to his poem, while the neoclassical poets studied classical form, Whitman walked among the people, describing their shared identity in free verse.
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.