When we think about the tone of a given work of literature we are referring to the attitude the writer takes about a subject or character, normally produced by word choice. When we try to work out the tone of a poem such as this one, it is important to try and establish the theme and message as this obviously helps us.
Clearly in this poem the tone of the speaker at the beginning is very frustrated as he listens to this reductionist, rational lecture that "explains" the cosmos and removes all mystery and wonder from contemplating the night sky. Words such as "add, divide and measure" perhaps express the frustration of the speaker as he listens to this lecture that everyone else is enjoying so much. His emotional reaction is clear when he says "I became tired and sick." As he moves outside, the tone moves to one of reverence and mystery as he moves into the "mystical moist night-air" and regards the stars "in perfect silence," in contrast to the lecture of the astronomer. The reverence implied indicates that any attempt to calculate the cosmos is doomed to fail, as it robs the galaxy of its mystery.