Evening civil twilight is the time between sunset and when the sun is 6° below the celestial horizon; morning civil twilight is from the sun being 6º below the horizon until the sun is on the horizon (sunrise).  

The air almanac refers to the ‘upper limb’ (top edge) of the sun but nautical almanacs refer to the centre of the sun and then a correction is made for the sun’s ‘semi-diameter’ (radius).