In mid-afternoon, a fast moving silent dull white sphere moves erratically and flies away very fast.We were around the pool, in the middle of the afternoon. Around 28 Celsius, almost no clouds. All of a sudden, we noticed an white spherical object that seemed to be about right above us. It moved erratically for around 20 seconds (back and forth, in circle)and then seemingly acquired a steady high velocity (no apparent acceleration; so it must have been extremely fast)and went north-east. Didn't see anything after that. Perspective might have been wrong, but it seemed to be at an altitude of around 1 Km, maybe more; but not much more, because there was a line plane nearby and it was obviously much higher than the object. Judging by the looks of it, the object's speed must have been of several hundred Km/h. If my estimation of the height is correct, the object must have been around 5-20 meters in diameter. We didn't hear a sound. The object didn't seem to emit any light. It looked like a dull white sphere to us, but it's possible it was of slightly different! shape. It didn't emit anything like flames, smoke or any distortion, like an animal, a plane or a balloon would. This leaves 2 possibilities to me: one, a UFO, and two, a rather strange atmospheric anomaly. One thing is for sure, I had never seen anything like it in my life.