Sisters observe dark disc in sky over city park; sight landed saucer opening as they hide.

I was a teenage cashier and my sister a PBX operator in her early twenties. That evening I was looking up at a dark starry sky as we walked through Brentwood Park, facing south (toward town). I noticed a circular patch of the heavens which was totally black, devoid of stars. "Isn't that peculair?" I asked. "Must be a cloud," she replied.
We stood looking up less than a minute, then resumed our walk. After taking several steps, I stopped, too curious not to look again. I was struck by how much larger the black circle appeared, and now it seemed to be moving very slowly(silently) directly overhead -- TOWARD, NOT FROM the direction it had been moments before. This was headed south! I had to hold my head far back to see the whole thing. It looked huge, low as or lower than an airplane.
We walked to a bench near the brick building where we lived. The temperature was so pleasant, we decided to sit outside awhile, and talk. A full moon had risen and the night was now very bright. We decided to go inside, but as we stood to leave, I noticed -- about a block's distance south of us, in the shadow of several large trees (I think magnolias) there sat an unlit, domed saucer, not much larger than a volkswagon bus. It was sitting a couple feet above the ground on what appeared to be slender rods. It didn't look tall enough for people to stand in.
As I pointed this out to my sister, the left edge of the dull metallic gray craft began to move as if a separate portion of the rim was being raised. This was not hinged like a car's trunk; it moved more like a hydralic awning. My sister grabbed my arm and we crept backward into the building. The private screened porch was inset and we were afraid to go out on it for a look. The landing site was out of view from the windows inside the living room.
To my knowledge, neither of us saw the saucer/s again.
An afterthought: if I'd "dreamed up" the experience -- logically, the vehicle would have opened with a ramp moving ground-ward. (Of course, we didn't hang around long enough to watch the complete action, so there could have been more to it.)
Witness indicates that the date of the experience is approximate.  PD