Bright White Circular Disc descends along the outside of Chicago lake front hi-rise

It was a late summer afternoon, I was 19 years old and in my parents condo in Chicago, Lake Point Towers, about 15 floors up.  The picture windows faced out towards Lake Michigan & Navy Pier.  All of a sudden, their dog (small poodle), scrambled and jumped up on the ledge seat of one of the picture windows and started barking furiously.  When I ran into the room to see what was going on, I saw a very bright white flat circular disk @15ft in diameter, descending perpendicular to the ground, moving slowly down the outside wall of the hi-rise, about 10ft away from the building and window. I was stunned as I watched this object and the dog continued barking as it went directly by the window. I was absolutely amazed and in awe watching this thing. The air seemed electrified and humming, but no obvious sound emanated from this light disc.  I visually followed it as it slowly descended down the building and watched it hover out over the Lake, go around the building an! d out of sight.  I immediately ran (took the elevator) down to the lobby to see if I could still see it out over the lake, but by the time I got out to the street, it was gone. I asked the doorman if he'd seen the "light", but he had not.    I will never forget this experience and I have spoken of it to family and friends ever since...some believe me, but most dismiss it and laugh.  Bottom Line: I KNOW WHAT I SAW!!!   
Witness indicates that the date of the sighting is approximate.  PD