摘要：AbstractNaked eye observations of morning twilight phenomenon at Hail and other areas of Saudi Arabia are recorded. During the interval from 2014 to 2015, about 80 morning twilight observations were carried out in Hail at longitudeλ = 41°42′E and latitudeφ = 27°31′N for a desert background. The phenomena was followed over azimuth angles ranged from 0° to 20° of solar vertical direction and from 0° to 10° along the altitudinal range. By selecting 32 days with a very good visibility, it was found that Sun depression,Do, lies in a range between 13.48° and 14.69° with an average of 14.014° ±0.317. The difference between our obtained value and that one which is currently applicable in Saudi Arabia is about 4°. The results indicate that dawn (white thread browser) occurs at a sun vertical depression angleDo = 14.66° (mean + 2SD) according to the normal eye estimations. The results at different areas in the deep desert in KSA showed that the beginning of morning twilight and true dawn is at sun vertical depressionDo = 14.88° (mean + 1SD). The current study shows significant results, which are comparable with both naked eye observations and photoelectric measurements of true dawn in both Egypt and Libya for desert background.
