A large silent circular object hovered above our campsite blocking out 75% of the starry sky.

We were camping in the Sierra mountains near Mono Lake. We were laying on our backs looking at the stars. 
From a cliff 100 yards from our campsite a large 75 yards in diameter perfectly round object hovered from behind the cliff and moved directly above us. It hovered above us for about two minutes before hovering slowly back to behind the cliff. It was moving very slowly, 5-10 mph. 
We both saw the exact same thing. It perfectly siloutted the stars in a giant black circle. It covered about 75% of the sky, but it was not too far above the treetops. 
We had a very clear view of it but it was totally black so we could not make out its surface structure.
Date may be approximate, although the witness does not indicate that fact.  PD