The Determination of Dawn Time Based Sky Brightness Using Sky Quality Meter (SQM): A Case Study in Medan City

Vania Azzahra Lubis, Jihan Nafilah, Jihadin Jihad

Abstract


The dawn determination was empirically measured by measuring sky brightness with the Sky Quality Meter from November 1 to November 30, 2024, at the Islamic Astronomical Observatory of UMSU. Observations are made every one minute toward the eastern horizon, and the zenith is observed two hours before the scheduled Fajr hisab. The data are processed using the Moving Average method to reduce noise, allowing the pattern of brightness decline towards dawn to be consistently identified. The results show that the inflection point, a sharp change in the mag/arcsec² value, marks the dawn of the ṣādiq. It indicates that the increase in dawn light was detected at a shallower angle than the hisab criteria of the Indonesian Ministry of Religion, which set the beginning of Fajr at -20°, and the SQM data still show full night conditions without a significant increase in brightness. One of the data shows that the deepest dawn light can still be detected down to –18°, which represents the deepest limit of the local atmospheric response to sunlight in Medan before ṣādiq forms stably. However, the value of 18° does not appear consistently daily, so it cannot be used as the primary reference in determining the time of Fajr. This study confirms that photometric measurements using SQM and Moving Average analysis provide an objective approach to determining the onset of dawn and support the need to re-evaluate the criteria for national depression angles to better match the real dawn phenomenon in urban tropics.

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References


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DOI: https://doi.org/10.33096/jah.v2i4.27421

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