What's the reason the Astronomical twilight doesn't have constant time? What's the reason the Astronomical twilight doesn't have constant time although its degrees below the horizon (18 degrees) are constant?
Look at the extremes. At any time of the year, it takes only minutes (several minutes) for the Sun to sink well below the horizon at the equator. On the other hand, it can take days (several days) after the September Equinox for the Sun to sink well below the horizon for locations above the Arctic Circle. As an extreme extreme, at the South Pole, it takes multiple days for the Sun to rise from just the top of the Sun showing to the full Sun showing (or to set, six months later). Astronomical twilight lasts for several weeks at the South Pole.