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Variation of the evening zenith sky twilight brightness with sun's depression (D°) at Abu-Simble in blue, yellow and red colours.
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Most of the previously published results of the zenith sky twilight brightness, obtained by different investigators, are given in different units. These results have been converted to (mag/arcsec²) to make a comparison between them. The variation of the colour indices (B-V) and (B-R) of the zenith sky twilight with the sun’s depression will be stud...
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Citations
... The brightness of the sky in twilight is a function of the angle β. For instance, on the edge of civil twilight and nautical twilight, i.e., at β = 6°, the apparent brightness of the sky is 13.1 mag/arcsec 2 (Nawar et al., 2020). In our measurements, we gather flux using a circular aperture having a radius of 100 effective pixels (i.e., those emerging after the 2 × 2 binning). ...
Unpolarized sunlight is scattered by aerosols acquiring partial linear polarization. By aiming a ground-based detector vertically upward, it can record the polarimetric response of aerosols that are illuminated by the Sun. As the Sun sets, a portion of the sky is shadowed and the polarimetric response of the aerosols in the unshadowed region can be measured. This provides a means of scanning different portions of the atmospheric column with time. By comparing the measured polarimetric response with that of model agglomerated debris particles we can place constraints on the sizes and chemical composition of the aerosols in different portions of this column. We conducted a survey over 24 different epochs from April 2021 to December 2022, consisting of approximately 600 measurements of polarization of the atmosphere in twilight at the Ussuriysk Astrophysical Observatory. We found that most of the measurements correspond with water-ice particles or dust. However, on some occasions organic carbon dominated the measurements. These epochs correspond with increased fire seats in the region.
... We have yet assumed it to be achromatic (gray) just for simplicity. Twilight surface brightness values at zenith (I z in mag V /arcsec 2 ) as a function of solar angle θ s , averaged from available measured data are tabulated below [59][60]: Table 1 ...
... Morning zenith is brighter than the evening up to 1 mag V /arcsec 2 ; the reason may be the solar heating of the atmosphere during daylight, whereby the molecular density decreases in the evening [59]. The zenith surface brightness I z can be converted to zenith luminance Y z (kcd/m 2 ) as per the following equation [73]: ...
Besides the simulation of daylight luminous intensity distribution for interior architecture, spectral rendering of the sky hemisphere has been a rather important task for the video game developers. A veridical panorama under vigorous conditions in real-time requires a credible sky model as well as a prompt algorithm. The main purpose of this work is confined to the generation of an interactive WEB application which can run on any general purpose device lacking dedicated graphics processing but still can achieve sub-second response time. A convincing clear/turbid sky reproduction for both daylight and twilight was possible thanks to the employment of a single-pass semi-analytical model without the necessity of any pre-computing or recursive numerical integration effort. The highly parameterized structure enables to experiment many meteorological variations such as turbidity, altitude or ozone layer thickness. The shadow of the Earth has been empirically included into the algorithm in order to construct the Venus Belt. Some rendered images are compared against real photographs to evaluate the soundness of the proposed framework. The imitation of the Martian sky appearance has further been demonstrated in order to reflect the flexibility of the model to diverse atmospheric compositions.
This study is concerned with determining the altitude of the sun under the horizon to the morning and the evening of the night sky including the true dawn and dusk in cases moonless and moon light conditions. True dusk and dawn appear only after the total energy of the stars appears or disappears as an inevitable result of twilight light. The measured of the night sky are done for four months (from May to August, 2018) in Malaysia (3.82° N, and 100.8° E). The Sky Quality Meter (SQM) was used to measure the brightness of the night sky. The measurements were taken when the device was directed to the zenith position during the entire monitoring period. The two methods that calculate the threshold for eye visibility are M and M1 in the case of the cloudless conditions. The determination of true dusk and true dawn is based on the concept of the onset of the eye's visual threshold of magnitude (m, mag/arcsec2) as M= ∆m = 1.3m (which is the absence or appearance of the total energy of the stars relative to the celestial background), M1 =NELM=6m and the relationship to the sun vertical depression of Do (degree). At the Moonless, the depression of true dusk ranges as 11.2° ≤ Do ≤14.8°, while Do of the true dawn ranges as 12.2˚ ≤ Do ≤14.5˚. In case the full moon, the beginning of distinguishing dusk and dawn light from moonlight is Do ≈ 9.5˚ at 17.3m. The mean values of the light minimum of a full-moon-case, for morning and evening twilight is Do ≈12.5˚ at 17.9m and elongation ≈171˚. The average light level magnitude throughout the night is 20.61m (scotopic vision) and is at Do ≈15˚ in the beginning of night (or ends).
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.
This research is a new addition to the previous work done in Egypt to determine the exact time of the true dawn due to the urgent need of society for that. Twilight observations were carried out by SQM and the naked eye (N.E.) in the high visibility conditions for the morning twilight sky in the time interval (2018-2019) at Fayum in Egypt (29° 17´ N, 30° 03´ E, 50 m Elev.). The true dawn is found to be appear between Do≈14° to 14.8° according four different criteria applied in this work;(1) the threshold of eye in the magnitude, which is as 0.83 m from the full night (Abdel-Hadi and Hassan (2022) (I and II)), (2) the naked eye observations by the group, (3) the relation: H (Horizontal) =πZ (Zenith), (4) at 0.015 cd/m 2 as minimum energy for the mesopic region of twilight. The observations show that the clouds absorb about 19% of the energy (cd.m-2. degree-1) during the full night(Do=26.5˚-18˚), about 2.5% of the energy (cd.m-2 .degree-1) during the twilight period (Do=18˚-2.5˚) from the rays falling on.
Light pollution is an environmental problem, the consequences of which include higher level of night sky brightness, negative effects on organisms and natural ecosystems, degradation of quality of life and potential human health problems. Light pollution arises due to improper focusing of luminaires, excessive intensity of artificial light and lighting in unsuitable time. This diploma thesis focuses on changes in light pollution during the night. The main working hypothesis does not assume significant changes in light pollution during the night. The literature review of the diploma thesis describes natural and artificial influences on night sky brightness, temporal changes in light pollution, methods of visualization of light pollution in maps and characteristics of the study area. Within the study area in the Central Bohemia, there were executed measurements of night sky brightness using a Sky Quality Meter. The field stands have been chosen to represent urban, suburban and rural landscape. Based on the analysis of time series of night sky brightness, three types of night sky brightness courses were identified. The most intense changes of night sky brightness were proved in suburban landscape. On the contrary, the least intense changes of night sky brightness were proved in rural landscape. Afterwards, influences of light pollution and other natural and artificial factors on these observed changes of night sky brightness are discussed. The intensity of light pollution in the study area was vizualized using the interpolated map.
