Because of its size, the dish at Arecibo works in a slightly different way to normal radio telescopes, in that it is fixed within the ground and doesn't move about. It is able to see regions of the sky above it by having a moveable detector suspended 150 metres above the base of the dish. This 900 tonne platform is held aloft by eighteen cables and can be moved around by electric motors with millimetre precision.
In this way, the dish can receive signals from nearly a quarter of the visible sky, and not just straight up. Also, because the Earth rotates every 24 hours, the telescope can actually see a fairly large region of the sky throughout the day and night. Remember, radio telescopes can work during the daytime because they do not observe in normal light.