The poem is about a astronomer lecture.The narrator uses the preposition “When” in the title to create a future of wait. Another linguistic choice is the use of the verb “to hear” because it is a perception verb. Moreover it doesn't means “to listen” and the verb “to hear” explains most part of the poem. "Heard" is an important word because it is a simple past so it is totally over and it is a memory. The verb "learn'd" suggests that the astronomer is an expert.The poet writers the poem in the first person.

The poem is arranged in a single stanza but half of the text starts qith the word "when" and has the same structure: anaphoric structure. The effect of this choice is to underline the student bored. The poem is about an experience of a students who has hear an astronomer unwillingly. The text aren't a specific structure but there is free verse. This future of wait represents a figure of the speech: CLIMAX, an anaphoric expectation. In the first part of the poem the punctation has a function to recall the attention of reader in the first/last word of the line. The line lenght is a line longer and longer, so we have to read quickly and quickly.

The passive form of the verbs suggests that the narrator was subjected to proofs and figures.

At the same time the narrator uses always a scientific field. The text uses a passive form until “ measure them”. The poet with this anaphoric structure wants to suggest that the astronomer liked because everyone applauded at conferences.

With “how” the scene is concentrated in the astronomer. This poem is the memory of something pleasant happened previously. This repetitions in the text has a function to keep the level of wait and to show the monotony of the lesson.

The alliterations of the sound “s” have a sense of heaviness. The narrator is repress by proofs and figures.

The verb “to became” is the semantical conclusion of the tiredness of the protagonist. Anaphoric structure is repetitive and it gives veracity to the wat and to the boredom. The second part of the text  is liberating and emotional because the narrator find perfection in one night.

There are four semantic fields:

mathematics-geometry: proofs, figures, to range, columns, chart, diagrams, to add, to divide and to measure

astronomical: astronomer, to look up and stars

emotional: tired and sick

scholastic: to learn/to lecture, to heard, to show, lecture-room