Bismillah. I blogged about this last year. A number of people have asked me about these again, so here goes:
There are variations in UK mosque timetables for dawn and end of Suhur (Sehri) timings. The variation can be over one hour.
This is due to different methods of calculating the time of the first appearance of dawn, usually based on the sun’s angle below the horizon. Some jurists recommend 15 degrees, others use 18 or 17.5 degrees.
I use the visual observation of dawn, as per the original Prophetic practice. I don’t follow clocks rigidly to the minute – that is not the spirit of Islam. I don’t wear a watch any more, to reduce our over-obsession with telling time via clocks as opposed to our bodily rhythms and other cycles of nature. (I recommend “Pip Pip” by Jay Griffiths for a wonderful exploration of time, clocks, nature & humanity. The world’s biggest four-faced clock tower, currently being built opposite the outwardly light-polluted, but inwardly light-bathed, House of God and Symbol of the Heart in Mecca, really is a sign of the times!) But of course, in modern, clocks-dominated societies, calculated timetables are inevitable.
Visual observation of the dawn usually tallies with a 15 degree angle in urban or light-polluted areas and 18 degrees in very rural, dark-sky locations.
In high latitudes in the summer, the “dawn” is there all night, and the jurists use various rules such as: nearest valid time; most recent valid time; last fifth, sixth or seventh of the night, etc.
As with the Ramadan start date, these are all valid interpretations / ijtihads. Individuals and communities should follow what is most sensible for them.
Allah knows best!