 Return signals were received about 2.5 seconds later, the time required for the radio waves to make the round-trip journey from the Earth to the Moon and back.The receiver had to compensate for the Doppler shift in frequency of the reflected signal due to the Moon's orbital motion relative to the Earth's surface, which was different each day, so this motion had to be carefully calculated for each trial.The antenna could be rotated in azimuth only, so the attempt could be made only as the moon passed through the 15 degree wide beam at moonrise and moonset, as the antenna's elevation angle was horizontal.