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).