In the mystical moist night-air (line 7)
Whitman paints a vivid scene using only a few words. This is the kind of phrase you might find in a novel by Beat writers like Jack Kerouac and Allen Ginsberg (no coincidence, considering how much they were influenced by Whitman). The speaker no longer feels alienated from nature, as he did inside the lecture room.
Look'd up in perfect silence at the stars. (line 8)
The poem moves from the abstract, scholarly term "astronomer" in the title and the first line to the simple and concrete word "stars." "Astro-," of course, refers to stars. In the same way, the speaker discovers the superiority of a simple, direct relationship to nature.