When I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts and diagrams, to add, divide, and measure them,
When I sitting heard the astronomer where he lectured with much
Whitman is describing an astronomer, perhaps a teacher or a lecturer speaking to an audience or a class. Whitman’s use of the word “heard” implies a lack of attentiveness. He is just hearing the words the astronomer is saying, not actively listening and not paying heed. Also, describing the astronomer as “learn’d” is a somewhat back-handed compliment. “Learn’d” is a precisely chosen word and it shows that Whitman thinks it is impressive that the astronomer is so knowledgable, but in the end, Whitman finds true appreciation for beauty elsewhere. 

When I Heard the Learnd Astronomer