At the time the signal was heard, the Big Ear radio telescope was only adjustable for altitude (or height above the horizon), and used the spinning of the Earth to listen across the sky. Given how fast the Earth spins, and how big the telescope's observation window is, the Big Ear could listen to any given point in the sky for just 72 seconds. This means that a continuous extraterrestrial signal would only be able to be heard for exactly 72 seconds, and the recorded strength of such signal would display an increase for the first 36 seconds—peaking at the center of the observation window—and then a decrease as the telescope moved away from it. All these characteristics are present in the Wow! signal.