Let us remember that it can often be dangerous to look at sections of poems in isolation and not to look at them as a whole. It is difficult to understand the full meaning of the section of verse you have quoted because it acts as a contrast with the first part of the poem, which presents the calculating nature of scientific inquiry of all of its mathematical rationalism and glory. In response to such an excellent presentation of rationalism, which has effectively "measured" all that there is to know about the stars and the galaxy, the speaker finds himself feeling "tired and sick." Having the mystery of the universe robbed by such a scientific approach leads the speaker to feel rather the worse for wear. What "heals" him or restores his spirits is going out of the lecture hall and wandering in "the mystical moist night-air." The use of the word "mystical" is key here as it reminds us, and the speaker, that there is something mysterious about the night and the stars that no amount of science can ever fully explain. The "perfect silence" of the last line with which the speaker looked up at the stars again represents another contrast between the lecture of the "learn'd astronomer," which again points towards the limitation of knowledge and the way that science can never fully explain or capture the majesty of a night sky. Poe once wrote that science was a "vulture" that "preyed upon the hearts and minds of poets," and this poem captures this aspect of s science. The speaker is only able to restore himself by going out into nature himself.