determination of dawn is shown

determination of dawn is shown

The Effect of atmospheric humidity level to the determination of Islamic Fajr/morning prayer time and twilight appearance

Abstract. Islamic prayer times are based on the astronomical position of the Sun in the sky. One of them is the Fajr prayer. It is marked by some indicators in the morning twilight which is white light spread in the Eastern horizon. However, determining the true time of twilight can be difficult. One of the reasons is the effect of atmospheric humidity to the appearance of morning twilight. The higher the humidity, the redder twilight sky appearance. This paper discusses this effect. It is shown that despite of the same Sun’s position, sky color can vary considerably. Observations of various solar dip angle have been made to study this effect. Visibility for different angle can change accordingly. We obtained that the average solar dip for Fajr prayer by morning twilight images was -18°39'29.4''. 1.

Introduction: The twilight is divided into civil, nautical and astronomical twilight corresponding to solar elevation angle of -6, -12 and -18 degrees, respectively [1]. Islamic Fajr time is related to the dawn or morning twilight. However, no clear definition of the dawn. If it is interpreted as the time of the first light, then the dawn will correspond to -12 to -18° solar elevation angle, but it is not possible to be more precise. The rise of morning twilight for the beginning of fajr (namely the true fajr or al-fajr as-sadiq) could be discerned on the horizon by the appearance of a light, which ascends towards the sky and resembles a tail of the fox--this is known as al-fajr al-kazib (the false dawn). Then the light spreads on the horizon (and becomes like a white cotton) in such a way that whenever you look towards it. In other words, it is the spreading of the light on the horizon after it had been ascending towards the sky [2].
Dry aerosol particles scatter light, and this scattering increases above the deliquescence point, where solid particles absorb water and turn into fine droplets, at around 70-80% relative humidity. Hence, visibility may be low on days with high aerosol loading and when humidity is high. Since rain scavenges aerosol particles from the atmosphere, some of the best visibility days occur after strong rain events. Therefore, color appearance of the sky and transparency or visibility in the atmosphere could be connected to relative humidity, temperature, and atmospheric pressure [3][4]. The purpose of this paper is to determine the corresponding position of the sun in relation to the aforementioned definitions of dawn; and to construct color model examples at the time twilight appears, at various levels of atmospheric humidity.

Results and discussions Figure 1 shows the splendid colors of twilight with different measured relative humidity. Based on the image data, we found a relation between humidity and dawn color. From left to right, the humidities are 50%, 80%, and 95% respectively. The higher humidity, the more pronounced are the orange reddish colors. Colors are caused by scattering of sunlight by the particle and gas constituents in the atmosphere. Colors come about because the amount of scattering varies with wavelength. Since scattering characteristics depend on particle density, size and material, different atmospheric conditions are responsible for numerous varieties of twilight color displays. Determination of the time of true fajr (column 7) is obtained by images analysis with an indicator of light that spreads along the horizon. We have applied simple procedures to detect intensity variation near horizon from array of pixels as pixels profile.

Conclusions Based on the image analysis of morning twilight from several locations with different relative humidity, we can conclude that the higher humidity, the twilight sky colors became orange-reddish. Also, visibility was influenced by the level of humidity. The beginning of Fajr prayer can be derived by averaging solar elevation angle of defined the true dawn was -18°39'29.4''.
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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.

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