A 2 second exposure while rotating the camera. I used a Singh-Ray Vari-ND (variable neutral density; 2-8 stops) filter to get an exposure time long enough for this effect. Camera on tripod with a 75-300 lens at 75mm - the thing that made it easier was the lens has a tripod collar. I framed the image and rotated the camera before releasing the shutter just to be sure that no extraneous "stuff" was going to creep into the final shot. Once everything was checked I clicked the shutter with a remote release so as not to interfere with the rotation of the camera about the lens collar.
The "from camera" image was lightened (dodged) near the center and the saturation was increased (didn't have my "velvia sensor" loaded).