I would guess it's the fact that the farther side looks brighter than the nearer side. Animated 3D rotation without any shading or shadow always throws me off f

I would guess it's the fact that the farther side looks brighter than the nearer side. Animated 3D rotation without any shading or shadow always throws me off for a second. If the inside of the globe is slightly opaque, dimming the farther side proportionally, it might make it easier.