Whitman really isn't pleased with the astronomy lecture. We can sympathize, kind of, but we also think that there are worse things than spending a couple of hours in a room listening to a world-famous expert on celestial bodies. (For one thing, when he read the ad for the "Astronomy Lecture," what was he expecting: a laser show?) The speaker's reaction goes even beyond run-of-the-mill boredom. He feels nauseous and exhausted, as if the numbers the astronomer talks about were physical objects that assaulted his senses. The dissatisfaction in the first half of the poem sets up a contrast with the enormous feeling of relief and joy in the second.
The form and sound of the poem express the speaker's dissatisfaction using repetition and tautology.
The speaker has a physical reaction to the lecture because he does not separate the body from the soul.