How to determine “center” in a sphere? is there a grid to determine the center of a sphere or something thanks 
i’m learning new stuff
Center for using transpose lines? or for what you need the center?
the center when you started creating the sphere, when you rotate, i just want to know the front center of the sphere
If you’re using the transpose line, there’s a setting in Preferences for auto snap. If you’re displaying polyframes, you can see lines on your mesh which helps to pick the spot. Get the Axis plug to switch views, it’s helpful.