Sunset and twilight

Back to consideration of the sunset in Normandy. There are 24 x 60 = 1440 minutes in a day. The Earth rotates 360 degrees about its axis in 24 hours. That means it takes 1440/360 = 4 minutes per degree. The Sun subtends 0.5 degrees and thus takes 2 minutes from the bottom first touching the horizon until the tip disappears. Civil twilight is defined as lasting until the Sun is 6 degrees below the horizon https://en.wikipedia.org/wiki/Sunset That means 24 minutes. Apparently night starts when the Sun is 18 degrees below the horizon, so 72 minutes after sunset. I'll need to find out why the boundaries are set thus.