But the moon becomes invisible when it is a very narrow crescent. I saw the moon before sunrise on Dec. 11 and it was a pretty narrow crescent then...it was cloudy this morning so I did not attempt to see it, but it would have been narrower.
The moon rarely gets so thin that it’s invisible. But of course that depends on your eyesight. I can see it with my naked eye even when it’s really thin. Believe it or not, when I used to do it some years ago, I could actually see Venus in the daytime sky. And I confirmed it by then putting the binocs on it. I had to first have some general idea of where it was. If I didn’t have some general idea, I’d probably never find it.