well, assuming from the original set he must be somewhere in that building or somewhere like this building:

Makes some sense..

Also the buildings in the outside makes this building be really top from the others.

And, due to the lighting on all the object in the scenes, it's impossible to change the direction of the sun. But you forget that Bespin is a planet, but cloud city floats above it so therefore could possibly have a slight rotation to it making the direction of the sun the same at sunrise and sunset.

Assuming and only, cant be this fact established to the exterior shots?

-Angel