The title of the poem gives to the intelligent reader several information. The poem is a memory of a passed occurrence. This is suggested by the first word of the title: “when”. The verb “I heard” gives information about the setting of the events narrated in the poem: they took place in the past. The verb moreover let the reader understand that the poem is narrated in the first person.  
The second part of the title is dedicated to the character the narrator heard about. The character is an astronomer, so I expect the poem to be about astronomy. The attribute “learned” characterize the astronomer as an important and experienced one. He's not like the other ones, he's one of the best .

The poem is arranged in one big stanza of 8 lines. It has got free lines and all of these begin with the capital letter.

The repetition of the title at the first line increases the reader’s conjectures. The phrase is incomplete so the reader wants to know more, he wants to know what happened after the narrator heard the astronomer.

The description of the lesson consists of a list of actions separated by commas from one another: this creates a sense of gradual accumulation and suggests the lesson is heavy going, monotonous and tedious. This hypothesis is confirmed by the use of passive verbs: they recall the passivity of the narrator, who becomes just a bored person in the lecture room.

Moreover, the narrator uses the verb “to hear”, which is a perception verb, instead of “to listen”, in order underline that he/she didn’t pay attention to the lesson.

The anaphora of “when” keeps high the expectation of the reader who is waiting since the title to know what happened to the narrator.

The climax reaches its peak at line 5, where the narrator admits his/her feelings and finally reveals what happens: he/she feels sick and tired and runs away from the lecture-room. Now he's outside and it's dark. This fact gives information about the setting: it was night. Now he can walk around and finally look at the stars.

Even if the astronomer took an entire lesson about stars,the narrator was not attracted because he didn’t care to know the scientific and astronomy aspect, but just wanted to watch the stars. The anaphora of “when” conveys the monotony of the lesson, while the contrast between “much applause” and “perfect silence” conveys the different sensations the narrator felt going from the noisy room to the “mystical moist air”. The alliteration of “m” in these words suggest the sensation and the mood of peace and mystical calm the speaker felt going outside and looking at the stars.

The final alliteration of “s” in “silence... stars” conveys the deep feeling of peace and freedom the narrator finally felt.

The message of the text is that a scientist, using charts and diagrams, numbers and tools, can analyse the characteristics of a stars but can’t describe their eternal beauty.