Well, I’d rather have a “Really/that’s interesting” than an indistinct grunt and silence. It’s just an acknowledgement , most of the time. And I cannot fault even an earnest “really” or “are you sure”, the first or second time if I have not previously established what’s my cred on the subject at hand.
“I didn’t know that” I choose to take as a sign we’re making progress, the equivalent of “Ignorance fought”.
She did not see you looking it up. Being ignorant of astronomy, she is unaware that that not only can one accurately predict the apparent positions of celestial bodies years in advance, but that you need not be Carl Sagan to have learned to identify a particular syzygy. And what reason has she to take our word that it’s so?
I do sometimes start to get a bit short with others when I happen to know a hard fact and they insist in making it a debate – some time ago I realized it’s more like they seem puzzled that I sound so sure and certain I’m right.