Naked eye determination of the dawn for Sinai and Assiut of Egypt

Abstract Twilight observations were carried out by naked eye in the period (2010–2012) for north Sinai (Lat. 31°4′N, Long. 32°52′E) where the background is desert, and for Assiut (Lat. 27°10′N, Long. 31°10′E) in the period (2012–2014) where the background is agricultural land. The purpose of these observations is to calculate the depression of the sun below the horizon at which the normal eye can discriminate the dawn (morning white thread) for two sites. The results indicated that this discrimination takes place at vertical sun depression angles, D o = 14.61° and 13.665° at Sinai and Assiut respectively.