In the summer months in the UK there is a debate over when Fajr time starts.
The main cause of this is that the sun doesn't go far enough below the horizon for pitch black conditions to occur.
This causes a problem because Fajr time is supposed to be when the "true dawn" starts from pitch black conditions.
Based on the unique conditions of being at such a high latitude, different groups of scholars have come up with different solutions, the most conservative of which is to start Fajr in the middle of the night.
The main solutions are to either use a different point to decide when Fajr starts (hint: the "true dawn" was not the true test of fajr, but a test that worked where tested with the real requirements that were laid out in the Qur'an).
Currently, you will see different timetables in the UK, having fajr time start at just after 1, around half 2 or around quarter to 4. There are others, but these seem to be popular.
The latest time is AFAIK from the method that the night (sunset to sunrise) be split up into 7 segments, and the last is fajr.
The earliest time is explained by the idea that "since it never goes fully dark, and before the middle of the night it cannot be fajr, the safest bet is to consider Fajr time to start at midnight". I used to call this the midnight formula as a joke (hint: forget British Summer time and use at GMT). But now I am told this is called "Nisful Layl". Middle of the night.
I have never given much credence to this formula, but there was always a question over whether any changes occur in the night sky to indicate the start of Fajr time, that were conveniently being ignored by competing formulas that suggested that Fajr time started later on.
This night I spent between around 1am and 10 past 2am in the country side (just past the M62 towards Halifax in order to get past the motorway being a distraction to the east) to see if there is a change in the night sky that could be considered significant.
No noticeable change was observed with my naked eye.
While this is a singular observation, and the whole night was not spent, so as to confirm when Fajr time actually starts, its enough to put doubts over the midnight formula. Others are free to carry out their own observations - all that is required is a location with little artificial light and eyes.
(Maybe the more scientific could take long exposure pictures of the horizon in order to detect changes.)