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

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 thirddegree 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°b elow the horizon. ARTICLE HISTORY

Pengamatan Fajar Ṣādiq Menggunakan Sistem Otomatisasi Observasi Fajar Dan Sky Quality Meter Stand Alone Di Sulawesi Selatan (Studi Komparatif)

“…13 Di lokasi lain, Noor & Hamdani (2018) melakukan observasi pada periode Agustus hingga September menggunakan SQM di Pantai Tayu Pati dengan kesimpulan bahwa fajar terdeteksi ketika matahari berada pada -17°. 14 Sementara itu, selama 26 hari dari bulan Juni dan Juli 2015, dengan kembali menggunakan SQM, Saksono & Fulazzaky (2020) menyimpulkan bahwa fajar adalah terdeteksi saat Matahari berada pada -14° ± 0,6 di Depok. 15 Rohmah (2016) melakukan penelitian selama 22 hari di Juwiring, Pati, Yogyakarta (Jawa Tengah), Kaibon (Jawa Timur), dan Bandung (Jawa Barat) dengan menggunakan kamera digital.…”

Ideal Data to Determine Accurate Fajr Time

“…Sky Quality Meter (SQM) is a simple and very easy to use photometric tool for measuring sky brightness levels. SQM is produced by Unihedron, and has been widely used for sky quality mapping research [23], [24] determine astronomical site [25], [26] , light pollution research [25], [27]- [29] research in the health sector [28] , behavioural research birds [30], [31] , to research on determining the time prayer [2], [8], [13], [15] . SQM uses a light frequency sensor TSL237, SQM in this study is set to quantify in mpsas every 1 minute with the direction of the sensor to the zenith [32].…”

“…In Indonesia, the elevation angle of the Sun used for the beginning of subuh prayer time is -20° or zenith=110° [2], lately this standard has been questioned because it is assumed that when the Sun is in that position, the dawn light on the western horizon has not yet appeared. Dawn light research has been carried out with various approaches, some using the naked eye [3] , using a digital camera [4][5] , and some using simple photometric tools, namely Sky Quality Meter (SQM) [2], [6]- [8].…”

“…There are several techniques used to determine inflection points, including using polynomial functions as has been done by [8] , with this method, the determination of the inflection point uses a degree of polynomial that varies from polynomial of degree 3, polynomial of degree 4, polynomial degree 5, to polynomial degree 6, depending on the SQM data itself, even several times have to reduce unnecessary data. In addition, in the research conducted by Saksono, et al used less ideal data because the data used came from a location have sky quality below 20mpsas or fall into category 5 and above on the bortle scale [16], [17] .…”