Why does east and west appear to be mirrored in the celestial sphere? I'm wondering why East is left and West on the right of the celestial sphere if you look at images of it?
Example below:

(source)
You're viewing the celestial sphere from the inside looking outwards, as opposed to maps of the Earth's surface which are viewed from the outside looking inwards.

Imagine drawing a map on a sheet of glass. When you view the map from the other side, it will appear mirrored.