Cairo University

MTPR Journal


Studying the difference between solar and astronomical twilight using image analysis

Ramy Mawad 1, Magda Farghaly 2, A. H. Hassan 2 , Walid Abdel-Sattat 3, Hussien M. Farid 3, Shahinaz Yousef 3, M. Amin 3
1. Astronomy & Meteorology Department, Faculty of Science, Al-Azhar University,
Nasr City, Cairo 11488, Egypt.
2. Department of solar and Space researches, National Researches Institute for Astronomy and Geophysics
(NRIAG), Helwan, Egypt.
3. Astronomy, Space Science and Meteorology Department, Faculty of Science, Cairo University,
Giza, 12613, Egypt

Vol./Issue: 19 , id: 275

Studying the types of observed twilights according to image analysis taken by Canon EOS 20D camera were carried out during 13-16 Jan 2016 in Aswan Egypt (desert land). It is found that there are two types of twilights. A) Astronomical twilight called also Pseudo-Dawn (non-thread bright): caused by Zodiacal light or the solar radiation reflection from the asteroid belt, and interplanetary matter when the solar depression is approximately in the range 19.5°-16.5° (18° in average). It is not appeared in all days of our study, it appeared only in days 14 and 15 when solar depression angles were -17.54° and -18.88° respectively. B) Solar twilight (morning white thread): produced from the solar radiation directly at the depression angle of the Sun is nearby 15° before sunrise and 16° after sunset. Solar twilight occurred when the Sun's depression angle at -15.56° -15.56°, 14.93°, -15.38°, -14.53° in the current studying period. Starting time of the solar twilight is slightly different from naked eye, about ~1°. It is found that the astronomical twilight starts brighter and vertically wider than solar twilight. Astronomical twilight starts at altitude 8° with vertical thickness ~30°, while solar twilight starts at altitude ~3° with thickness about 1-2° only. The solar twilight of the morning can be divided into three stages: a) Darkness (Fajr) occurs at a depression angle of the Sun nearby change to about (16° - 12°). b) Semi-darkness stage (Ghalas): in-between stage, occurs at a depression angle nearby change to about (12° - 5°). Twilight has a grey color. c) Lightning stage (Isfar): pre-rising, occurs at a depression angle nearby change to about (5°-0°) with reddish twilight color. Because of the difference in mesospheric thickness, temperature and composition at sunrise and sunset occurrence, twilight color starts white for sunrise twilight and ends red for sunset twilight.