I was walking my dog at about 10pm. It was a very clear June night in 1991. The stars were bright in spite of the light glow from downtown which is less than two miles (straight line) away. We had left our home and walked north across the street and then turned right, downhill and east for about 135 feet to the corner. While my dog was busy smelling about, I looked about the neighborhood to the north, east, and south.
While looking south, a strange, out of place darkness caught my attention. The normally bright sky was missing and I remember staring at this "void" sky and wondering why I couldn't see the clouds. Then I saw the movement.
This darkness had a shape which I could see only when the end of it passed over me toward the south south-east. It was moving slowly at a steady pace. There was no noise and my dog did not become aware of it.
The rear edge became clear and sharp as the bright sky appeared behind it. The shape revealed to me was circular and its thickness was defined by a clear, straight, vertical line which connected the two circular edges which I assumed to be the top and bottom of the shape.
Using basic triangular math, this object, if at an altitude of 500 ft. had to be 500+ ft. in diameter. It "felt" to me that the object was about 800-1,000 ft. high, which would increase the object's dimensions greatly.
A personal word: I am male, and at the time of this observance, I was 54 years old and in great health. My eyesight was above average at 20/10 vision. As a member of the U.S. Army I served in Korea during 1954-56. I became familiar with daytime and nighttime flights of the Air Force near the border (DMZ) with North Korea.
After the military, I worked for a large civilian airline (TWA) in Chicago, both at Midway and O'Hare airports. I worked at O'Hare when the Boeing 707 was introduced to commercial flight. During those years I worked the night shift exclusively. I was able to discern the types of aircraft in flight at night, as did the majority of my fellow employees.
The strangest part of this to me is that I did not become excited! My dog and I merely crossed the street and went home. I mentioned it to no one, not even my wife. I never completely forgot the experience but it didn't seem important until I was at that corner again with my neighbors, months later, and I felt excitement for the first time when I was telling them about it.
I should have been shouting about this to the rooftops! Several of my friends, TWA pilots and crew, had privately shared tales of UFO's back in the late '50's, so I knew of UFO's and SHOULD have been quite excited, but at the time it felt just "ordinary"!