Sun Vertical Depressions and Their Effects on the Morning Twilight Phases in Egypt

B. A. Marzouk, Nasser M. Ahmed, K. A. Edris, R. A. Mawad, M. M. Beheary, A. Bakry, A. H. Ibrahim, A. R. Mouner, Ahmed M. Abdelbar, A. Abulwfa, Samy A. Khalil, M. G. Rashed, Yasser A. Abdel-Hadi, A. H. Hassan, A. E. Ghitas, M. A. El-Sadek, U. A. Rahoma, M. A. Semeida, G. A. Goma, M. Sedek, R. H. Hamid, S. S. Khodairy, S. Hamzawy, E.Abdel-Wahed, M. M. Hussien, M. A. Shahat, A. A. Elminawy, Hussien M. Farid, W. A. Rahoma, M. Yossef, K. Ali Eden

Springer Proceedings in Physics · 2025

Abstract

This research is a new addition to the previous work carried out in Egypt to determine the altitude of the sun (sun vertical depression) corresponding to the beginning of the twilight. The observations of this issue began since more than 60 years in our institute and were attended by many astronomers and astrophysicists. Naked-eye (N.E) twilight observations of morning twilight were carried out in good viewing conditions by four instruments: two digital cameras, CCD cameras and SQM during the time interval (Aug. 2015–Dec. 2019) in many regions in Egypt by several research groups. The regions in Egypt were: Kottamia observatory, Kharga, Aswan, Hurghada, Marsa-Alam and Fayum. More than 30 scientific observers and amateurs participated in these observations. The purpose of these measurements was to determine the sun vertical depression of the sun below the horizon, D o ( D o = -altitude of the sun) at which the normal eye can discriminate the true dawn (strong horizontal white thread). The results of these measurements indicated that the true dawn by the naked eye was observed at sun vertical depression D o = 14.56° (mean + 1SD)), while it was ranged from D o = 14° to 15° by measuring the light intensity using multi instruments depending on three different criteria for eye threshold of M, M1 and M2 (in the moonless sky conditions). The full hierarchical shape of the false dawn does not occur regularly in every morning. Generally, the interval time of the false dawn was found to be within 15° ≤ D o ≤ 18°. In the most morning twilight days, the percentage of color portion was Blue > Green > Red before D o ≈ 13.5° and it changes its behavior after this degree.

Keywords

How to cite

Marzouk, B. A., Ahmed, N. M., Edris, K. A., Mawad, R. A., Beheary, M. M., Bakry, A., Ibrahim, A. H., Mouner, A. R., Abdelbar, A. M., Abulwfa, A., Khalil, S. A., Rashed, M. G., Abdel-Hadi, Y. A., Hassan, A. H., Ghitas, A. E., El-Sadek, M. A., Rahoma, U. A., Semeida, M. A., Goma, G. A., et al. (2025). Sun Vertical Depressions and Their Effects on the Morning Twilight Phases in Egypt. Proceedings of the 14th Arabic Conference of the Arab Union for Astronomy and Space Sciences, 178–209. https://doi.org/10.1007/978-981-96-3276-3_14