Considering the title, the intelligent reader should be curious to find out/discover what happen "when I heard the learn'd astronomer". In that the message seems to be incomplete. It goes without say that the poem may concern - involve an astronomer and it tells about a memory because the title is in the simple past. Also the text is probably a narrative text where a first person narrator tells about his own experience. In addition/beside/ furthermore a close reading of the semantic choices makes the reader wonder why the speaking voice uses the verb "to hear" instead of to listen to. It must to be said that to hear does not necessarily involve a will it is a persuasive verb rather than an action verb. Besides the reader can realize 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 verses but punctuation seems to 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. Line length is compact in the last four lines and differently from the previous ones it returns the reader an idea of stability and balance. As a reside the reader's maybe eager to find out the reason why there is such difference.         
      Structure analysis elause the understanding of such choice: the first six lines introduce the event told by the narrator while the remaining four lines express the narrator emotional reactions to the experience he lived. The poem is the record of a narrator experience when he was a student and went to an astronomy lecture held by an expert in astronomy. The narrator tells the learned astronomer used the typical tools and resources of scientific subjects: "proof, figures, colons, charts" and the language used reminds the typical micro language of Mathematics. You can therefore realize that the poem exploits the semantic field of math, geometry, physic to convey the message. Indeed the verbs are the ones of calculations. In addition, the speaking voice tells that the expert used all his wit and he received lot of applause 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 how he felt and he does that using the language of emotions.         
      Interesting is to notice the narrator uses the time adverb soon immediately follow by the phrase "an 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 that different from the audience that much appreciate the lecture the narrator felt bored, tired and sick. Probably he didn't feel much involved in the experience: to tell the truth it sounds as if he could not stand it any longer his reaction was he rose and escape the room to went outside in the "mystical moist night-air".     

     The alliterative use of the "m" sound conveys the interesting feeling of the night air as the intelligent can understand from the adjective mystical, ones generally used by the religious author.
In short, you can understand the astronomy that was really able to drown the narrator interested was "the perfect silence" in which he could look the stars. The stars and astronomy all together can better be known by a personal emotion experience rather than by a lecture packed with notions, diagrams, or another mathematical language, the narrator seems suggest.   
      The anaphoric use of the language in the first part of the text recreate (on the sound level) the repetitive effect of the learned astronomer lecture at the same time keeping curiosity alert so that the reader may still be curious to find out what happen. 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 bored and the final escape coincide with the freedom of looking at the mystical, pleasure landscape of a starring night.