Great painters know how to use just a few dabs of paint to create a setting. A few brushstrokes can indicate sunlight on a person's face, or trees in the distance. A lesser painter, on the other hand, might crowd the canvas with too much detail.
In this poem, Whitman uses just a few details to flesh out a full, vivid scene. The first half of the poem takes place in a cramped, suffocated lecture room. Equations and figures crowd around the speaker like those math lessons on Sesame Street where numbers speed around the screen, bumping into characters and generally getting in the way. (Don't tell us you don't remember your Sesame Street!) The room is filled with people who have come to learn about the stars – or at least theories about the stars – but you get the sense that they are only clapping because they have heard that the guy giving the lecture is a real smarty-pants. The repetition of the word "lecture" in line four is all it takes to bring back all those memories of sitting in a really dull class and waiting for the bell to ring.
In the second half of the poem, the scene opens up. The speaker seems to "rise" up almost like smoke or vapor and just waft out of the crowded room. The poem concerns nature, but we don't get a lot of flowery descriptions of plants, stars, and the air. We know that the setting suddenly goes from noisy to quiet as the speaker goes off by himself. The "moist" air leads us to think of a dewy summer night, with dark plants respiring in the shadows. The sky is clear and the stars are shining brightly. And, like a painting, the whole scene ties together into a unity, implied by the word "mystical."