Fast moving large circular object appears in early evening sky in Northern California.I was sitting on the patio of the community building in my apartment complex, looking at stars and a few airplanes descend or ascend. This is approximately 50 miles north of the San Francisco airport. I looked at the sky straight ahead of me and saw a round object approximately half to one-third the size of the moon, moving very fast in what appeared to be a westward direction. It disappeared behind trees, which there are a fair amount in this area. I heard no sound. The most startling aspect was the speed with which this object moved.