I think I found my own answer.
Declared objects don't have center vectors but bounding boxes. to find the
center of the Sun object in the above code I've applied the following
#declare SunMin = min_extent ( Sun );
#declare SunMax = max_extent ( Sun );
#declare SunVector =
<(SunMin.x+SunMax.x)/2,(SunMin.y+SunMax.y)/2,(SunMin.z+SunMax.z)/2>;
Now to sweat the next problem!
