On September 14th, 2015 I was on 7th ave at w20th st in Manhattan . The weather was perfectly clear, no clouds, blue fall sky. At approx 1pm I noticed a bright reflection from a jet as it flew from north to south over west Manhattan, a common sight as planes climb after leaving Laguardia. As it passed overhead I noticed an oddly bright white sphere moving slowly at very high altitude. No blinking lights, like a ping pong ball, just hanging there. It seemed to move slowly side to side, but very still when it stopped. I assumed I was seeing a large high altitude balloon when suddenly it moved in a wide arc very rapidly across the sky. It was very strange because as it moved it remained flat bright white. I lost view for a second and then saw it at a much higher altitude seconds later moving north west at very high speed. I am fairly familiar with aircraft and have never seen one move like this object.