الفهرس 
Chapter One : IntroductionOnly 14 pages are availabe for public view
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Abstract
The study of twilight brightness is very important to know more details about the optical properties of the Earth’s atmosphere. Besides its importance to fix the times and dates of some Islamic events such as the determination of the beginning of lunar Arabic months and determining the times of Isha and Fager prayers. Due to the lake of the studies of twilight brightness especially in ultraviolet and infrared colors, the photoelectric observations of zenith sky twilight brightness are carried out in the present study for the first time at Kottamia observatory in Egypt.
The measurements have been obtained using a new full automatic photometer. This device is constructed mainly to measure the intensity of intense light of twilight and the faint light of different component of the night sky on the same foot of equality. Five wide band glass filters U B V R I, have been used in observations. The measurements have been converted from photon to mag/arcsec2. The gained results are tabulated and graphically represented on figures, and show the relation between zenith sky twilight brightness expressed in mag/arcsec2 and sun’s depression (D°).
The results show that there is a decrease in zenith sky twilight brightness with the sun’s depression (D°). For the sun’s depression between 0° and 3° the brightness decrease slowly, while for sun’s depression between 3° and 10° the brightness is linearly decreased. These results have been obtained for all colors. Also, the higher values have been illustrated for IR color and the lower values for UV color. The results at Kottamia observatory in ultraviolet and infrared, as it is mentioned before, have been obtained photoelectrically for the first time in this study.
The obtained results of zenith sky twilight brightness were compared with the results published by different investigators at the areas of Helwan, Daraw, and Abu-Simble in Egypt after converting them from S10(λ) to mag/arcsec2. Also, the results published in Maryland by Koomen et al., (1952) have been converted from candle/foot2 to mag/arcsec2. The comparison shows that there are differences in zenith sky twilight brightness from site to another. These differences are due to different factors affecting on the sky twilight brightness such as geographical latitude, elevation above sea level, solar activity, seasonal and atmospheric conditions. The present results have been also compared with CCD camera results published by Patat et al., (2006) in Chile.
The values of zenith sky twilight brightness expressed in mag/arcsec2 at different colors have been used to deduce the color indices (U-B), (B-V), and (V-I). The color indices are drawn as functions of sun’s depression (D°), and it is found that the maximum reddening in the sky during the twilight at sun’s depression between 1° and 5°. It is also found that the sky tends to blue color between sun’s depression 5° and 10°. For the sun’s depression (more than 10°) the sky becomes relatively red.
The results of the present work demonstrate that there is a need for more observations for sky twilight brightness at different positions in the sky. This is to study the effect of different factors on the twilight brightness and colors.