the learn'd astronomer; (line 1)
The speaker expresses his dissatisfaction using irony, which is when you say something other than what you mean. Sarcasm is a common form of irony. He doesn't really think that the astronomer is so "learn'd," at least concerning the important things in life.
When the proofs, the figures, were ranged in columns before me; (line 2)
The phrase "before me" really signals his dissatisfaction here. He feels under siege by all those intimidating-looking numbers. They seem directed at him and not just the crowd.
How soon, unaccountable, I became tired and sick; (line 5)
You might think that his dissatisfaction could be easily explained: he's bored! But Whitman doesn't want us to leap to this simple conclusion. He says that his negative reaction is "unaccountable" – basically, a mystery. Notice that his unhappiness manifests itself in physical symptoms.