Considering the title the intelligent reader should be curious to find out/to discover what happened "When I heard the learn'd astronomer" in that (nel senso che) the message seems to be uncomplete.
It goes without saying that the poem may concern/ involve an astronomer and that it tells about a memory because the title uses a verb in the Simple Past. Also the text is probably a narrative text where a first person narrator tells about an experience of his. In addition/besides / furthermore a close reading of the semantic choices makes wonder why the speaking voice uses the verb "to hear" instead of "to listen to". It must be said that to hear does not necessarily involve a will by the listener since it is a perception verb rather than an action verb. Besides the reader can realize that the title coincides with the first line of the poem and, as a consequence, the choice reinforces the importance of the line and the question implied in the title.
The layout makes clear the poem is arranged into free verse but punctuation seems organize the story told into different moments.       
A careful, more precise look will show the reader there is only one full stop at the end of the text.
The line length is compact in the last four lines and, differently from the previously one, it returns the reader an idea of stability and balance.  As a result the reader may be eager to find out the reason why there is such difference.
Structural analysis allows the understanding of such choice: the first five lines introduce the event told by the narrator, while the remaining four lines express the narrator's emotional reaction to the experience he lived.
The poem is the record of a narrator's experience when he was a student and went to an astronomy lecture held by an expert in Astronomy ("the learned astronomer"). The narrator tells the learned astronomer used the typical tools and resources of scientific subjects: "proofs", "figures", "columns", "charts" and the language used reminds the typical micro language of Mathematics. You can therefore realize that the poem exploits the semantic field of maths, geometry, and physics to convey the message. Indeed the verbs are the ones of calculations ("to add", "to divide", and "to measure"). In addition, the speaking voice tells that the expert used all his wit and received lots of applauses by the audience there but never in the first five lines does he hint at what he felt like.  It is only in the last four lines (the ones that have mostly the same length) that he refers to how he felt and he does that using the language of emotions as you can see from the exclamative pronoun "How".         
Interesting it is to notice the narrator uses the time adverb "soon" immediately followed by the phrase "unaccountable " that means that he himself does not know why he had such a reaction: he " became tired and sick" in the space of a little time.       
The reader now can understand had differently from the audience that had much appreciated the lecture, the narrator feel bored, tired and sick. Probably he did not feel much involved in the experience - to tell the truth it sounds as if he could not stand it any longer. His reaction was the rose and escaped the room to wander outside in "the mystical moist night air". The alliterative use of the "m" sound conveys/ communicates the interesting feeling of the night air as the intelligent reader can understand from the adjective mystical, one generally used by the religious code.    
In short, you can understand the astronomy that was really able to draw the narrator's interest was "the perfect silence" in which he could look at the stars.   
The stars and astronomy all together can better be know by a personal  emotional experience rather than by a lecture packed with notions, diagrams or any other mathematical language, the narrator seems to suggest.
The anaphoric use of language in the first part of the text , recreates (at the sound level) the repetitive effect of the learned astronomer's lecture, at the same time keeping curiosity alert so that the reader may still be curious to find out what happened.  This also explains for the existence of only one full stop in the whole text.
In conclusion all levels of connotation perfectly integrate to convey the message of the narrator ‘s first moment of boredom and the final escape  coinciding with the freedom of looking at the mystical pleasure  landscape of a starry night.