The Polar twilight and its presence for around 6 weeks after the Equinox at the North and South Poles is a separate issue with a separate rotational cause.

The variations is twilight length at habitable latitudes and especially the difference between Equatorial latitudes and those further North occupy the theme and resolution here - the slower the latitudinal speed away from the Equator, the longer the twilight when the Sun comes into view and turns out of sight each 24 hour day.

The present explanations using the angle of the descent of the Sun is so pathetic that those who follow them should be allowed to be left to their own devices and not troubled but neither should they interfere in the proper explanation.

The distance from horizon to horizon or indeed the circumference surrounding an observer, at least at sea level, is only a number of miles so the Sun coming into view at one horizon and disappearing from view at the opposite horizon can be unhelpful when demonstrating the global view of twilight/dawn lengths and their variations across latitudes with different rotational speeds.

As the Sun disappears from view, the astronomer should look in the opposite direction as their location approaches and turns through the circle of illumination. By looking towards the right (as the Northern hemisphere observer faces the Equator ) and ignoring the Sun, the observer can now concentrate on how fast they turn through the circle of illumination. Presently the Moon and Mars provide a wonderful reference as the contrast becomes sharper as the distance of observer and their location to the circle of illumination increases after they pass through the terminator.

http://en.es-static.us/upl/2018/10/2...mars-pluto.jpg


It is also why the transition from light to darkness is more rapid at the Equator than at any other location North or South as any person who travels between latitudes will know. The leap from local horizon to global perspective takes a bit of effort but the value is not in the direction of the setting Sun as regards twilight lengths but as a location approaches the fixed circle of illumination daily.