General
Anyone who advocates a hard fixed time for Subh al-Sadiq (dawn) or Isha, regardless of the formula used to determine those times (i.e. 18°) , has not truly understood the number of variables (e.g. climate) which can made a significant difference from one day to the next. The best any project or formula can do is establish a band of 'approximation' based on an calculuted upper and lower limit. Additional steps like taking the more conservative prayer times will mitigate the risk. Any method, approach or formula which gave Subh al-Sadiq (dawn) and Isha prayer times within that established upper and lower limits would be considered valid and can be followed if it supported local observations.
In establishing the realistic band where Subh al-Sadiq (dawn) or Isha seems to be occurring, the depression angle is not near 18°. That is to say the sky remains dark after the sun reaches the depression angle of 18° when trying to establish Subh al-Sadiq (dawn). For Isha, the sky becomes dark much earlier than 18°.
The strongest factor when establishing Subh al-Sadiq (dawn) or Isha is not the depression angle of the sun but rate of the depression change; the period of time it takes for the sun to travel from one degree to the next. For locations closer to the equator the rate of change remains relatively constant. In Mekka, the transit of the sun between one degree takes 5 minutes and this is constant all year round. In the UK, the time for the sun to transit one degree is 6 minutes in the winter and 16 minutes at the height of summer.
The Sky Quality Meter (SQM) is a powerful tool for measuring light changes and opens the potential for assigning a value to what is observable dawn (Subh al-Sadiq). As it is more sensitive than the human eye, and so allows you know when Subh al-Sadiq (dawn) is not occurring and when Isha (Shafaq al-Abyad) has occurred regardless of the depression angle of the sun.
Fajr
The SQM was able to show the changes in light levels as the sun approached the horizon but it took sustained observations to map the observed dawn (Subh al-Sadiq) with an MPAS reading (19.5)
When mapped to the depression angle of the sun, the calculated Fajr (Subh al-Sadiq) time set by this project was between 14.77° in the winter and 12.69° in the summer. One of the potential reasons for the smaller depression angle in the summer is the warmth and humidity of the day and the shortness of the night.
The SQM readings established the point at which the sky first started to get lighter, often before the visual perception of the human eye. Even at the extreme definition of dawn, the SQM data was able to determine a time for Fajr even in the summer months when the night is very short.
Isha
The project did not set out to establish the time for Isha but the SQM has provide a unique opportunity to measure light changes whether that be leading up to sunrise for Fajr or after sunset for Isha.
The changes in light levels as the sun drops below the horizon after sunset has been measured but as yet on 'fixed' MPAS reading has been established which corresponds to Shafaq al-Ammar. Observations have supported the decisions around the MPAS reading after which no red light remains (Shafaq al-Ammar) being around 16 MPAS. With the sky being dark enough for white light to have disappeared by 19 MPAS (Shafaq al-Abyad), it has been possible to construct a Isha timetable taking advantage of the flexibility given in Fiqh. The SQM supports the calculation of an Isha time which uses just Shafaq al-Abyad all year round.
When mapped to the depression angle of the sun, the calculated Isha time set by this project was between 12.92° in the winter and 8.35° in the summer. One of the potential reasons for what can be considered a low depression angle of the sun when establishing Isha is the impact of climate - twilight is being observed after the warm and humidity of the day leading up to sunset.
The SQM readings established the point at which the sky did not get any darker, often beyond the visual perception of the human eye. Even at the extreme definition of Isha, the SQM data was able to determine a time for Isha contrary to the belief of some that Isha does not occur in the UK during height of the summer months.
Persistent Twilight
Persistent twilight is a scientific principle and a function of the depression angle you wish to use but this does not necessarily translate to visible twilight remaining. For example, if you choose to use 18° then for 72 day of the year in Leicester during the summer you have persistent twilight. If you choose to use 13° you have zero days.
The Sky Quality Meter (SQM) has clearly shown that during the summer months after the sun stops reaching a depression angle of 18°, you still get many days when there is no influence (in terms of twilight) from the sun as it transits below the horizon. It is only after you reach the period where the sun does not go below 14° do you start to see a correlation between the transit of the sun and the resulting light levels (twilight).
It is definitely the case for Leicester that no alternative Islamic method of calculation for Subh al-Sadiq (dawn) is required (e.g Nisf al-Layl) as there is no cross-over between the extreme point of when Isha can occur and the earliest point when Fajr can occur.