Large black, circular U.F.O. in the night sky in the Summer of '89.

At age 15 (currently 41) I saw a large circular U.F.O. in my old hometown of Walterboro, SC.
It was the summer of 1989 (in June or July--possibly even August--b/c June 23rd was the premier of the Batman movie) and one night my brother and I decided to go outside and play w/ the glow-in-the-dark Batman Frisbee that we’d just gotten (“Bat Fever” was sweeping the nation at the time and merchandise was EVERYWHERE).  We walked out into the back yard (somewhere between 7 and 9 PM I’d guess) and started throwing it back and forth.
Our town is small and produces very little light pollution, so on a clear night one can see stars in every direction.  That night was clear and moonless and I believe the only light we had on was a small flood light on the back of the house, but we may have unscrewed the bulb to make it completely dark--I really can’t recall. Anyway, we’re tossing the disc around and at one point I jumped up to catch a high toss and as I did I saw, slightly to my left, a large black flying object that had a perfectly round bottom, no lights and made no sound.  It was so black it blended in w/ the night sky, and the only reason I even saw it was b/c it blotted out the stars as it flew by.
 I’m not good with height, but I’d say it was somewhere between 300 to 600 feet in the air and I judge its size to be about half the size of a football field based on the small patch of wooded area [that separated our neighborhood from the next] that it flew over.  It had to be traveling at least 100 MPH or more b/c it was gone from sight in roughly 5 seconds.  I just stood there until it was gone and I may have asked my brother if he’d seen it (I don’t think he did), but honestly that’s all I can remember after so many years.
To this day I have no idea what it was. I’m telling this story in the hopes that someone else in the area saw the same thing and will also tell their story. It was an amazing thing to see.
Witness indicates that the date of the sighting is approximate.  PD