I don't know where they are originally defined, but these definitions are given in astronomy and navigation texts, In celestrial navigation, star sights are commonly taken between civil and nautical twilight as both the the star and the horizon must be visible (normally navigational stars are limited to a selection from the list of 57 "bright stars").
The above calculations use the (center of the) geometric sun and neglect refraction of earth's atmosphere. Calculated sunrise and sunset are calculated for 54' (arcminutes) below the horizon under the same conditions. The 54' figure is the sum of the mean semi-diameter and mean refraction at the horizon. These both vary slightly with conditions but on average place the upper limb at the apparent horizon. I think the definitions originally come from navigators and astronomers, but civil authorities have generally accepted the ones for sunrise, sunset, and civil twilight, for rules regarding headlights, etc. ________________________________ From: "[email protected]" <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Sun, June 27, 2010 1:12:56 AM Subject: [USMA:47979] OFF TOPIC: Twilight I don't often do this, but this group is so good about precise definitions, I couldn't resist when I ran across this just now (and had no idea these existed or where they are defined): Civil Twilight Civil twilight is defined when the sun is 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities. Nautical Twilight Nautical twilight is defined when the sun is 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct. Astronomical Twilight Astronomical twilight is defined when the sun is 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible. Ezra
