On Mark Twain forum, Taylor Roberts raises the question of whether or not Mark Twain witnessed either the 1874 or 1882 transit of Venus. While I don’t know the answer to that question, I noted in response that he did mention the transit of Venus in a September 30, 1876 speech. Here’s the relevant quote from a New York Times transcript.
(W)hen you come to our civil service we serenely fill great numbers of our minor public offices with ignoramuses; we put the vast business of a custom house in the hands of a flathead who does not know a bill of lading from a transit of Venus (laughter and a pause)—never having heard of either of them before (laughter);
You can read the whole speech in an explanatory note at the Mark Twain Project site or at Mark Twain Quotes.
Twain also mentioned the phenomena in his “Some Learned Fables, For Good Old Boys And Girls” of 1875.
Further discussion followed, and then the united voice of the commission begged Lord Longlegs to speak. He said:
“Fellow-scientists, it is my belief that we have witnessed a thing which has occurred in perfection but once before in the knowledge of created beings. It is a phenomenon of inconceivable importance and interest, view it as one may, but its interest to us is vastly heightened by an added knowledge of its nature which no scholar has heretofore possessed or even suspected. This great marvel which we have just witnessed, fellow-savants (it almost takes my breath away), is nothing less than the transit of Venus!”
Twain was surely interested in and knew about of 1874 transit of Venus even if he may not have witnessed it firsthand.