Pseudo dawn and true dawn observations by naked eye in Egypt

Abstract

Naked eye observations of the morning twilight phenomenon, beginning, end of a pseudo dawn and the true dawn are recorded. These results are an average of 38 observations carried out at Wadi Al Natron (30° 30′ N, 30° 09′ E) with a desert background during the years 2014–2015. These observations enabled us to get an average depression of the sun below the horizon of about 19.74° (begin of zodiacal light), 15.41° (end of zodiacal light) and the beginning of twilight (true dawn) is 14.57° (The highest value of confidence, mean + 1SD) which lies in a range between (12.48° ≤ Do ≤ 15.14°). These results show that there is about 5° difference between our obtained value and that currently used in Egypt which is 19.5°. The azimuthally range of observations in the solar vertical direction ranged from 0° to 20°, but this phenomenon was followed from 0° to 10° along the altitudinal range. These results indicate that the currently used angle for calculating dawn in Egypt is the first zodiacal light (pseudo dawn) and not true dawn. Keywords: Pseudo dawn, True dawn, Naked eye observations