Single Light UFO moving over Los Angeles, Accelerated at unbelievably fast speed and dissapeared over the Horizon.

Light was directly overhead.  Was not able to estimate distance from ground.  We observed light moving against clear black star covered sky.  We thought that the light was an air plane but it stopped moving and made a circular motion.  We then thought it may be a helicopter but we were not sure if they could fly in a tight circle, we did not think so.  We kept our eyes fixed on the light.  It started moving in a northern direction and accelerated and dissappeard over the Santa Monica Mountains so quickly, less than a second that it was quite disturbing, to me at least.  I estimate the distance to be more than 50 miles or more.  
I don't know what it was but I knew even then that nothing could move so fast and so far in such a short time.