DETERMINATION OF THE TRUE DAWN BY SEVERAL DIFFERENT WAYS AT FAYUM IN EGYPT

This research is a new addition to the previous work done in Egypt to determine the exact time of the true dawn due to the urgent need of society for that. Twilight observations were carried out by SQM and the naked eye (N.E.) in the high visibility conditions for the morning twilight sky in the time interval (2018-2019) at Fayum in Egypt (29° 17´ N, 30° 03´ E, 50 m Elev.). The true dawn is found to be appear between Do≈14° to 14.8° according four different criteria applied in this work;(1) the threshold of eye in the magnitude, which is as 0.83m from the full night (Abdel-Hadi and Hassan (2022) (I and II)), (2) the naked eye observations by the group, (3) the relation: H (Horizontal) =πZ (Zenith), (4) at 0.015 cd/m2 as minimum energy for the mesopic region of twilight. The observations show that the clouds absorb about 19% of the energy (cd.m-2 . degree-1 ) during the full night(Do=26.5˚-18˚), about 2.5% of the energy (cd.m2 .degree-1 ) during the twilight period (Do=18˚- 2.5˚) from the rays falling on.