Time verification of twilight begin and end at Matrouh of Egypt

Abstract Twilight observations were carried out in the period «1983–1985» in different seasons at different sites mainly to deduce the beginning and the end of twilight. One of these sites was Matrouh. These observations were gathered using a refractor (an alt-azimuth scanner) of 10 cm diameter associated with a photoelectric tube of the type ( φ  →  Y  − 51) using 3-filters namely; the blue, visual and red filters. The photoelectric system was calibrated with the international photoelectric color system. The phenomena were followed in steps each 10° in both altitude and azimuth. The depression of the sun below the horizon was calculated from the local time of each scan. The results indicated (at a  = 5° and A  = 10°) for the latitude of Matrouh that the end and the beginning of the twilight occurred when the depression of the sun ( D o ) was between 18° and 20°, while the dawn times were found to be between 14° and 16°. The brightness of the twilight phenomena was followed, however, to different altitudes and azimuths [0° ⩽  a ° ⩽ 60° and 0° ⩽  A ° ⩽ 60°].