A big, black, round, & spinning UFO with a WAVY EDGE slowly moving across the sky in a straight line in Japan.

It was probably the fall of 1976.  I was 11 years old living in Japan by the Sagami bay in the city of Fujisawa in Kanagawa prefecture, about 10 minutes from the beach by walking.  (I am half Japanese, half American, currently living in America.)  
It was in the afternoon, probably around 16:00.  I was at the living room of my house, feeling dull.  I got the inclination to go to the back yard and breathe some fresh air.  So I did and looked up at the clear sky.  The UFO was slowing moving in a straight line in the direction of the ocean.  The direction was south west or south south west.  It was about 90 degrees above the ground (I looked right up at the sky).  There was no sound.  
The object was pitch BLACK, no shine; a "mat finish."   Its apparent size was that of a FULL MOON, so it was quite big.  The shape was unique and unusual.  It was like a dinner plate with a WAVY EDGE.  Imagine a sine wave with eight to ten cycles and make it into a circle/loop.  And the object was SPINNING fairly quickly. 
I observed it until I could no longer see it.  The timing of my sighting was impeccable.  I went to my back yard at the perfect moment.  
In hindsight, I wonder if I was telepathically called me to come out of the house.  (Just a speculation.)
Witness indicates that the date of the event is approximate.  PD