Large Orange Circlular Object Moving Slowly and Silently

I was outside of our home, in the residential neighborhood of Montgomery Village, Santa Rosa, California.  It was about 9pm, in the fall, September.  I was pacing back and forth next to the sidewalk, speaking with my girlfriend, who was sitting on the curb.  As I turned and faced north, out of the corner of my eye, I saw a light in the sky.  I turned and faced it and pointed at it and said "Look!".  My girlfriend jumped up and we both watched it for about 10 seconds.  Then I ran to the front door of my home and yelled at my family to come see.  They were slow to respond.  By the time I went back to my original position to look at it, it was gone.  My girlfriend said it went over the small mountains to the south east.
 Its appearance - flawlessly circular, burnt orange, constant hue with small mechanical like random pulsations of equal size on its surface.  Moving slowly in a straight line, silent!   Size difficult to determine, because of it's circular shape, and dark blue background of the clear night sky.  My best estimate - between 40 to 200feet dia, approx. altitude of 400 to 1000ft, approx. speed  40-50 mph. Direction - north north east to south south west.  About 10 degrees off of horizontal, decline.
 The most striking characteristic of this object was:  silent operation.  No sound whatever.  Also, the appearance of the random pulsations against the burnt orange surface.  And it's apparent size in the sky.  About 20% larger than a full moon.
Date is approximate.  PD