In the title of the poem,  composed of an eight line stanza, the reader finds a rhyme between verb “heard”, which is an involuntary action, and the adjective “learn’d”. This rhyme marks the difference between the people who hears without knowing a thing about a topic and an astronomer who has studied and talks about the topic.

Another interesting aspect is the repetition of the adverb “when” in the first four lines. It arranges the poem in two parts: in the first one Whitman is listening to the lesson of the astronomer and he uses words regarding science and maths (e.g. “proofs” and “divide”). Another important aspect is the passive form of the verbs which enlighten the passivity of the poet and in general of the public who just listen to the astronomer without adopting the topic.

In the second part instead he goes outside and watches the stars and the language changes: Whitman uses words expressing feelings, like “tired and sick” and the verbs are in the active form. Further more Whitman uses the word “mystical” which expresses the unknown.

So between  the first and the second part there is a contrast: on one side there is the study of nature which is certain, while in other one there are feelings which are relative and the mystic element present in nature and unknown to human.

In this way the poet focus on the contrast between rationality (science in the poem) and feelings, in other words, between Enlightenment and Romanticism. Since Walt Whitman was a Romantic, he is soon bored by the diagrams, the proofs and the charts because science can only report calculation and data: it takes away from nature its beauty which can be seen only with human’s eyes. Plus, science describes nature quantitatively while poetry can describe its allure and what humans feel by watching it.

Regarding  the language, it’s quite simple and the reader doesn’t  find any particular difficulties but it’s required a knowledge about Enlightenment and Romanticism to be able to analyse the poem.