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New Moon’s Visibility Criterion Based on Photometric Data

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Schaefer (1991) determined the Danjon limit or minimum angle between the Sun and the Moon from which the Moon can be seen shortly after the conjunction. Schaefer's method uses Hapke's (1984) lunar photometric theory and considers a fixed value for the threshold illuminance. We show Schaefer's method and its shortcomings, and we expose a modified theory, where the threshold illuminance to see the lunar crescent depends on several factors, mainly atmospheric absorption. We consider that vision is a probabilistic phenomenon; that is, when we use the experimental data of Blackwell (1946), we cannot be sure whether or not the Moon will be seen. Finally, we conclude that «perhaps» Hapke's theory overestimates the shielding of the sun's rays by the irregularities of the lunar surface at large phase angles.
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In this work, an analytical study had been made of sky brightness variations using a photometer of Sky Quality Meter (SQM-LU). The sky brightness measurements were performed in Baghdad city of Iraq (Latitude: 33°20’N, Longitude: 44°23’E, and Altitude: 34m above sea-level). The values of sky brightness level were collected on 27 December 2019 as well as 10 January 2020 at sunrise and sunset. The aim of this research is to study on sky illumination contrast by taking moonlight (moonless/full moon) as a reference source. Additionally is to localize the direction of the sun during both sunrise and sunset. The results indicated that, in case of sunrise and moonless, the sky brightness level was obtained averagely at 13.60 ± 0.26 magnitude/arcsecond ² with sun’s altitude angle at -9.79° ± 0.12°. While, in case of sunrise and full moon, the sky brightness level was obtained averagely at 12.75 ± 0.25 magnitude/arcsecond ² with sun’s altitude angle at -8.87° ± 0.11°. Findings also showed that, in case of sunset and moonless, the sky brightness level was obtained averagely at 17.44 ± 0.26 magnitude/arcsecond ² with sun’s altitude angle at -10.66° ± 0.12°. While, in case of sunset and full moon, the sky brightness level was obtained averagely at 15.69 ± 0.25 magnitude/arcsecond ² with sun’s altitude angle at -10.25° ± 0.11°. In conclusion, the study shows a correlation between the sky brightness magnitude against sun’s altitude angle, moon’s phase, and weather conditions as well.
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