teen girl alone by stable sees dark grey hovering sphere

I was alone filling a bucket of water for a horse and noticed a dark grey sphere hovering stationery maybe 50 ft above the ground. I couldn't tell its size, or really its altitude.  I addressed it, i.e. introduced myself, and asked it to wait. 
I then ran up the hill to my house from the stable, and checked back a couple times to see if it was still there and it was until just before i entered the house.  After I went up two flights of stairs and looked out a big picture window, having gotten my parents attention, it was gone. 
I must have been about 14 at the time, maybe 15.  At the time I was a high school student at a prestigious Southern California Prep School, and an award winning equestrian.  I was mostly interested in science and wanted to become an astronaut. 
My freshman year at Stanford I became ill and changed majors from physics to Values Technology and Society, and ultimately became a doctor.  I now practice ob/gyn.  
I have never seen any other UFO.
Witness indicates that the date of the sighting is approximate.  PD