18 degrees is the basis for the calendar of The Presidency of Religious Affairs. However this is complied in Ramadans only. Out of Ramadan, fajr time adhan is usually recited 1 hour before fajr prayer. Following links provide further information on this topic:
Dear brothers and sisters at Islam & Quran
I would like to ask about fajr praying times in Istanbul and the calculation method used.
I recently vistied Istanbul and noted that the fajr praying occurs later than expected or calculated by a software I use in my Iphone.
My phone uses a sun inclination degree of 16 to calculate alfajr praying but I didn’t hear the Azan until 15 minutes after my Iphone announcement.
This occured over few nights.
Could you advcie me about this and also what sun inclination / depression used in Istanbul.