The tone of this poem, for the most part, is overwhelmingly weary. The word "overwhelming," indeed, is very apt here: Whitman employs parallel structures and repetition ("When..." "When..." "When...") to create a sense of monotony and convey how little interest he really feels in what "the learn'd astronomer" has to say. The impression the poem gives is one of being confined, like something meant to be calculated: "to add, divide, and measure." The poet can only endure so much of this before he states that he "became tired and sick."
The shift in tone between the first core section of the poem and the final three lines is almost palpable. From being "tired and sick," the speaker is then "rising," "gliding," as soon as he leaves the lecture room. The reader can almost feel the relief as the speaker breaks out into the "mystical night air," the repetitive drone of the calculations in the lecture room disappearing, to be replaced by the simple joy of being "by myself," "in perfect silence." There is no doubt that to look "in perfect silence at the stars" is more rewarding, in the opinion of this poem's speaker, than to be confined to a classroom attempting to analyze "proofs."