A number of alternatives to establishing Subh al-Sadiq (dawn) at 18° have been established using different methods over the years and here is the comparison between the light levels changes established using the Sky Quality Meter (SQM) and two commonly used systems used in the UK.
OpenFajr
A year long projects conducted in Birmingham, UK used a specialist "all-night camera" to record low ambient light variances at the horizon and supported by naked eye observations. The resulting dawn timetable is widely used across Birmingham.
MoonSighting.Com
A decade long research by Moonsighting.com describe the Subh al-Sadiq (dawn) as functions of latitude and seasons (day number of the solar year). All collected observations from different latitudes were plotted against day number of the year. With curve-fit technique, moonsighting.com came up with a function of latitude and seasons for Fajr (& Isha). There is no indication of any scientific equipment used.
The following charts compare the dawn times given by OpenFajr and MoonSighting.Com with the light levels recorded in magnitudes per square arcsecond (MPAS) by the SQM. The established dawn times by this project are also shown.
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During the first quarter (Jan to Mar) of year, the dawn times given by both OpenFajr and Moonsighting are around 19.5 MPAS (Grey into Dark Blue Area on the chart) and in-line with the observations and data of this project.
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During the first part of the second quarter (Apr to June) of year, the light level reading for observable dawn match very closely to both OpenFajr and Moonsighting. During the height of the summer at the solstice, OpenFajr and the light level readings of this project are comparable with Moonsighting showing a more liberal result when compared to MPAS reading during this time. For the remaining quarter there is a very close match between the two methods and our data.
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During the first part of the third quarter (Jul to Sept) of year, Moonsighting continues to give a more liberal result when compared to the MPAS reading during this time. During the height of the summer at the solstice, OpenFajr and the light level readings of this project are comparable. For the remaining quarter there is a very close match between the two methods and our data.
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For the last quarter (Oct to Dec), both OpenFajr and Moonsighting give dawn times around 19.5 MPAS (Grey into Dark Blue Area on the chart) and in-line with the observations and data of this project.
What we learn from the comparison:
- Three different methods (our light level analysis, use of a all-night camera (OpenFajr) and long term observations across seasons and latitudes (Moonsighting) all give dawn time with the same band of time
- All three methods give dawn time after 18°