It is this.

I could understand, when looking through a lens at an object, why we get only its reversed image--a simple law of optics explains _that_.

Therefore, in a map of the Moon, as the bottom means the north and the top the south, why does not the right mean the west and the left the east?
I suppose I could have made this out by a little thought, but thinking, that is reflection, not being my forte, it is the last thing I ever care to do.

Barbican, throw me a word or two on the subject." "I can see what troubles you," answered Barbican, "but I can also see that one moment's reflection would have put an end to your perplexity.
On ordinary maps of the Earth's surface when the north is the top, the right hand must be the east, the left hand the west, and so on.

That is simply because we look _down_ from _above_.