I've been told by an astronomer in past, that the appearance of the twilight isn't constant in the interval before the sunrise. That is to say that not every day the earth is going at the same speed, so for one day it takes 70 minutes for the earth to move from 18 degrees under the horizon (=astronomical twilight) to 0 degrees (horizon), and on other days it may be even 85 minutes. It depends on the speed of the earth circles the Sun. If what I've been told is true, then I'm looking for a way to calculate every day the accurate time between 18 degrees to 0.
The length of Astronomical twilight isn't constant, but the main reason is the tilt of the Earth, and not the orbital speed of the Earth.
The change from day to night is a result of the Earth spinning. In summer the sun is high in the Northern Sky, as a result its angle relative to the horizon when it sets is shallow and twilight will take longer. Back when I lived in York, twilight was permanent in June. The sky was still slightly pale at midnight.
This image show the day of the year from left to right and time from midnight to midnight vertically. In the centre you can see that in York truely dark skies are never achieved at Midsummer.
The best way to find the length of twilight at anytime and date is to use a service like time and date|. The calculations are not impossible, but not trivial either. Alternatively astronomical software such as skyfield can calculate twilight times too.