Night
One explanation but there may be many.
'Sunset is generally considered to take place when the top of the Suns disk passes below the horison.
Since the atmosphere scatters sunlight, the sky does not become dark instantly at sunset; there is a period of twilight.
During civil twilight, it is still light enough to carry on ordinary activities out-of-doors; this continues until the Sun's altitude is minus 6°.
During nautical twilight, it is dark enough to see the brighter stars, but still light enough to see the horizon, enabling sailors to measure stellar altitudes for navigation; this continues until the Sun's altitude is minus 12°.
During astronomical twilight, the sky is still too light for making reliable astronomical observations; this continues until the Sun's altitude is minus 18°. Once the Sun is more than 18° below the horizon, we have astronomical darkness. The same pattern of twilights repeats, in reverse, before sunrise.'
The calculations within the almanac used for aviation appear to be calculated using only Lat and Long and take no consideration of altitude. Maybe somebody thought of this and therefore, for training purposes, considered that the actual visual conditions should be taken into account, in order that a realistic visual experience of night was always encounted.
Hopefully an expert with the full knowledge of the history will chip in and illuminate us.
Last edited by homeguard; 20th January 2005 at 23:48.