I had just got my lawnmower out of the shed to do some mowing, it was a warm and cloudless day. I looked up towards the south west to admire the blue sky when I noticed an airliner flying in a westerly direction at what I would estimate as between twenty and thirty thousand feet. A small metallic sphere appeared at the tail of the aircraft, hovered for a few seconds and then slowly made it's way along the length of the fuselage, after it moved past the nose of the aircraft it disappeared. The sighting lasted no more than 15-20 seconds. I have been unable to come up with a rational explanation as to what I saw.