Predicting the accurate period of true dawn using a third-degree polynomial model

Abstract

ABSTRACT Sky brightness data have been widely used to document the light pollution and to localise the direction of the sun during both dawn and dusk twilight. The use of the most common data analysis techniques of graphical interpretation can give less accurate final results. This study used a Sky Quality Metre to record the brightness of night sky in mag arcsec−2 at Depok city of Indonesia during 26 days for the months of June and July 2015. Analytical technique of third-degree polynomial was established to predict dawn astronomical twilight and provides a good fit to the experimental data. The modelling results have been experimentally verified that astronomical twilight in the morning at Depok city with its environment begins when the sun is at the position of −14° ± 0.6 below the eastern horizon and ends at sunrise. In conclusion, the Muslim communities of Indonesia begin their dawn prayer 6° too early and have been found to calculate the true dawn incorrectly when the centre of the sun is at a depression angle of −20° below the horizon.