UFO Sighting

So I was outside this evening, and saw what just had to be an alien spacecraft. After hearing so many descriptions of UFO behaviour, this had to be my first experience. There was a large, semi-crescent shaped object, probably a hundred miles wide, that just hovered in the sky. It faded out, in a way that no human vehicle could, then suddenly re-appeared about an hour later. It seemed to grow rapidly in size, probably flying quickly in my direction at many hundred of miles an hour. It made absolutely no sound, and the whole experience was creepy. Totally unexplainable.